qtui: Set currentIndex on position update
qtui: Implemented basic FilterInput class.
qtui: Implemented filtering on treeView (but underneath model still requires to be updated after filtering)

Conflicts:
	src/qtui/playlist.cc
qtui: Fixed issue with filtered model mapping. It will now correctly map filtered model to Audacious model underneath.
qtui: Cosmetic fixes.
qtui: Cleanups
qtui: Fix scrolling to current playing song
qtui: Create Playlist::playCurrentIndex ()
qtui: Inherit Playlist widget directly from QTreeView