Fix loading of MicroDVD subtitles after 519ef2671600df1e7db79448754152e628df2f8e.

This commit fixes #2991.
Some testing on SetAlwaysOnTop. (not working)
PPageExternalFilters: cosmetics.

* Use more meaningful functions names.
* Use correct visibility for the members variables and functions.
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.