Qt: companion driver cleanup pass

Refactor pass on ui/drivers/ui_qt.{cpp,h} and ui_qt_widgets.{cpp,h}
to reduce Qt-specific surface, lift pure-C work out of C++ syntax,
and use libretro-common idioms in place of locally-rolled equivalents.

  ui/drivers/ui_qt.cpp           5620 -> 5462    (-158)
  ui/drivers/ui_qt_widgets.cpp   8888 -> 8783    (-105)
  ui/drivers/ui_qt.h              761 ->  747    ( -14)
  ui/drivers/ui_qt_widgets.h     1442 -> 1381    ( -61)
                                                 -----
  TOTAL                         16711 -> 16373   (-338)

No functional change. Build verified for ui_qt.o, ui_qt_widgets.o,
moc_ui_qt.o and moc_ui_qt_widgets.o; the only warnings emitted are
the same three pre-existing -Wdeprecated-declarations warnings the
original tree emits (QDir::operator=(const QString&),
QByteArray::append(const QString&), QListWidget::isItemHidden).

Iterative-build wall clock on the four targets drops ~9%
(28.1s -> 25.6s, hyperfine n=5), MOC-generated .cpp drops ~9%
(8666 -> 7874 lines), and total object size drops ~4%
(2.05 MiB -> 1.97 MiB).

Changes:

* Drop string_split_to_qt and use QString::split. The function was
  declared inside RETRO_BEGIN_DECLS - i.e. with C linkage - but
  returned QStringList, which is not a C-compatible type. The four
  call sites become QString::split('|') / .split(' ').

* Replace MainWindow::getScrubbedString with a static scrub_qstring()
  helper using string_replace_all_chars from libretro-common. Moves
  the function out of MOC (one fewer slot to generate).

* Collapse 11 trivial OptionsPage subclasses (MenuSoundsPage,
  UpdaterPage, QuickMenuPage, DriversPage, DirectoryPage,
  ConfigurationPage, CorePage, LoggingPage, AIServicePage,
  FrameThrottlePage, RewindPage) into one runtime-parameterised
  SimplePage class. Each was a do-nothing constructor plus a
  one-line widget() returning create_widget(DISPLAYLIST_X). The 12
  instantiation sites in *Category::pages() become
  new SimplePage(DISPLAYLIST_X, MENU_ENUM_LABEL_VALUE_X, this).

* Extract MainWindow::getCoreInfo() into a C helper qt_core_info_collect
  that emits two parallel struct string_list outputs (keys, values)
  with a status code (enum qt_core_info_row_status) packed into
  values->elems[i].attr.i. The walking logic is now pure C using
  libretro-common idioms (string_list_*, core_info_*,
  fill_pathname_basedir, string_count_occurrences_single_character);
  the C++ wrapper is a ~50-line loop applying status-driven HTML/CSS
  styling for firmware rows. Both consumers
  (onLaunchWithComboBoxIndexChanged, CoreInfoDialog::showCoreInfo)
  work unchanged.

* Replace four parallel thumbnail paths with arrays. The
  m_thumbnailPixmap{,2,3,4} pointers, onResizeThumbnail{One,Two,
  Three,Four} slots, and thumbnailChanged{,2,3,4} signals are
  consolidated into m_thumbnailPixmaps[4] and three static tables.
  The four thumbnailChanged* signals were never emitted and never
  connected anywhere - deleted. The four onResizeThumbnail* slots
  were one-line wrappers around setThumbnail("thumbnailN", ...) -
  deleted, callers use setThumbnail directly. Destructor,
  onCurrentItemChanged and onThumbnailDropped collapse to short
  loops; an explicit ThumbnailType -> widget-index mapping handles
  the asymmetric ordering between the enum (BOXART, SCREENSHOT,
  TITLE_SCREEN, LOGO) and the 1..4 widget naming (boxart, title,
  screenshot, logo).

* Convert MainWindow::getSelectedCore (which returned
  QHash<QString,QString> but only ever set a single field) to
  getSelectedCorePath returning QString. Sole caller updated.

* Replace QFileInfo/QDir/QFile usage with file_path.h /
  file_stream.h equivalents in three high-leverage spots:

  - renamePlaylistItem: now uses path_basename, path_basedir,
    path_get_extension, fill_pathname_slash,
    string_is_equal_case_insensitive, and filestream_rename.
  - onExtractArchive's per-entry delete-or-rename loop: now uses
    filestream_exists, filestream_delete, filestream_rename. Also
    fixes a pre-existing leak: the original code returned -1 on
    three error paths without freeing file_list.
  - loadContent's extension-filter block: collapses lastIndexOf('.')
    + .mid() onto path_get_extension, which the same block was
    already using three lines below.
  - Three QFileInfo(pathData).suffix() callers in
    add-files-to-playlist code: replaced with
    QString::fromUtf8(path_get_extension(pathData)).

* Split ui_companion_qt_init from a 516-line monolith into six
  focused static helpers (qt_companion_build_menubar,
  qt_companion_build_browser_dock, qt_companion_build_thumbnail_docks,
  qt_companion_build_core_selection_dock, qt_companion_restore_settings,
  qt_companion_select_initial_playlist) plus a ~75-line orchestrator.
  Behaviour preserved verbatim. The thumbnail-dock setup that was
  65 lines of 4-way copy-paste collapses to a 30-line loop using
  qt_thumbnail_widget_names from above.

Author: LibretroAdmin