PlayerSeekbar: utilize last WM_MOUSEMOVE point

In mouse capture mode, and seek only if new x coordinate differs.
It fixes madVR with delayed seek option enabled seeking to the same
point repeatedly.
PlayerSeekbar: code refactoring follow-up

A few minor things I missed in 444d7e9
Modify goto subtitle behavior

Jump only if there's a subtitle present. Previously mpc-hc performed 5s
jump if there was no subtitle, even beyond the end of stream.
New "Jump to Beginning" command