Many UI changes.

[dkosmari]

• The NavBar now spreads the buttons equally, to place the exit button at the very bottom.
• Show Themezer and local themes a grid (suggested by @Migushthe2nd).
• Local theme cells have a clickable star, to apply the theme directly from the grid.
• Redesigned theme details popup (suggested by @Migushthe2nd).
• Load thumbnails using the ImageLoader.
• Local themes and .utheme files are sorted alphabetically.
• Metadata is not installed separately from the theme anymore.
• Preview images are loaded from the theme folder, then looked up in the cache. Themezer is now bundling previews inside the .utheme, so there's no need to manage cached thumbnails.
• All themes are visible, even with no metadata. When no metadata.json is present, the only metadata available is the theme name (which is just the folder name).
• Theme details popup shows the list of files an installed theme contains.
• Installed themes metadata is loaded in a background thread.
• Quite a bit of code refactoring.

[dkosmari]

A small note, even though both the installed themes metadata and thumbnails are loaded in background threads, the UI still has some slowdown while images are being loaded. There's probably some synchronous I/O happening on the main thread, that's getting affected by it.