Add missing accelerators to mplayerc.rc

This commit fixes #2199.
Some testing on SetAlwaysOnTop. (not working)
External filters: Use two separated list for x86 and x64.

x86 filters only work with MPC-HC x86 and vice-versa.
Add some options to give a finer control on subtitles track selection.

The user is now free to choose if MPC-HC must prefer forced/default subtitles tracks (default is on) and if MPC-HC is allowed to change the subtitles track an external splitter selected to be the default (default is off).