[OxygenFallback] Correctly initialize fallback icon theme

For some reason, QIcon::hasThemeIcon does not work correctly.
[MainMenu] Create toolbars and sidebars menu to fix shortcuts

Shortcuts for sidebars are now available even before first showing
sidebars menu
[BookmarksToolbarButton] Final fix for rendering issues with various styles

It now works correctly with Windows, Gtk+, Oxygen and Fusion theme with
both Qt4 and Qt5.
The only theme that doesn't work correctly is Bespin, because it draws
icon size animation upon hover, but we are drawing the icon and text
manually. I think there is not much to do with it actually...
[DownloadItem] Add date of completed download

[ci skip]
[Translations] Try to load Qt (qt_*.qm) translations from Qt path

Also removed all qt_*.qm translations that are part of official Qt
distribution.