MainFrm: Prevent XySubFilter from being removed from the graph when cleaning up unconnected filters.

Temporary fix, either offer external subtitles as a pin on the graph or stop cleaning up the graph entirely.

Based on patch by Aleksoid.
Add AfxGetMainFrame() macro
PlayerSeekbar: code refactoring
PlayerSeekbar: smoother mouse seek
PlayerToolbar: be consistent with hand cursor

Previously it changed to arrow on right click