Populate the capture bar only when opening Capture mode.

Buggy systems tend to crash when listing the available codecs for the capture bar. With this commit buggy systems will still be buggy but MPC-HC won't crash unless the user really starts capture mode.

This commit fixes #3523 #3533 #3551.
Fix: Ensure that the "On Top" status is checked when losing the focus.

It used to be checked on being given the focus which is mostly useless.

This commit fixes #3541.