Qt: drop unused includes and split MainWindow constructor

* Drop three unused Qt includes:

  - ui_qt.h: QDialog, QStyledItemDelegate. Both are still needed in
    ui_qt_widgets.h (PlaylistEntryDialog inherits QDialog,
    ThumbnailDelegate inherits QStyledItemDelegate), and ui_qt.h
    pulls in ui_qt_widgets.h, so consumers continue to get them
    transitively for free.
  - ui_qt_widgets.h: QTabWidget. Not used in this header at all.

* Split MainWindow::MainWindow body into helpers. The constructor
  was 446 body lines of mixed widget creation, layout assembly,
  model setup, file-system configuration, dock setup, and signal
  wiring, with locals declared at the top and used hundreds of
  lines later. Five private methods now own the chunks that don't
  cross-depend:

  - setupPlaylistFooter() builds the zoom slider, view-type and
    thumbnail-type push buttons, items count label, and grid
    progress widget, attaching them under the playlist views.
  - setupModels() configures the playlist + filesystem proxy
    models and the table / file-table / grid views.
  - setupFileSystemBrowser() configures the QFileSystemModels
    (filters, root paths) and the directory tree view.
  - setupDockWidgets() owns the search / core-info / log dock
    widgets, including the searchResetButton that previously lived
    as a constructor local.
  - setupSignalConnections() owns the bulk of the QObject::connect
    block plus the m_dirTree initial-selection and m_thumbnailTimer
    setup that was interleaved with it.

  Constructor body: 446 -> 101 lines.

* Three small file-static helpers collapse repeated boilerplate:

  - qt_button_set_action_label() replaces the
    setDefaultAction(new QAction(msg_hash_to_str(label), btn)) +
    setFixedSize(sizeHint()) pair (5 sites).
  - qt_dock_add_to() replaces the
    addDockWidget(static_cast<Qt::DockWidgetArea>(
       dock->property("default_area").toInt()), dock)
    idiom (7 sites).
  - qt_dock_configure() replaces a setObjectName / setProperty(
    "default_area") / setProperty("menu_text") / setWidget
    four-line block (6 sites).

No behaviour change.

Author: LibretroAdmin