Qt(dialog provider): rework the part of popupmenu specific to skins2

Qt(dialog provider): rework the part of popupmenu specific to skins2

  - make it a bit simpler to read
  - add a shortcut to "Open skin.."
    (Ctrl-Shift-S chosen instead of Ctrl-S, since the latter
    is already used elsewhere)
  - Reuse the full version of "Open Media" with all accelerators
    being displayed instead of the light version
  - add the Help menu

With this new enriched popupmenu, skins2 should now offer all major
vlc hotkeys, which users have been asking for repeatedly.