Playlist: Scroll and select the latest added item.

This commit fixes #2987.
Playlist: Put MPC-HC on top and give the focus to the playlist after a drag-and drop.

Refs: #2987.
Some testing on SetAlwaysOnTop. (not working)
PPageExternalFilters: cosmetics.

* Use more meaningful functions names.
* Use correct visibility for the members variables and functions.
Fix a crash if VMR-7 (renderless) renderer failed loading.

This commit fixes #2992.