Replace commandline relative paths with absolute

Later we send the whole commandline through WM_COPYDATA,
and since current directories may differ, issues arise.

Fixes #3546
build.bat: Remove unneeded var after 84f536495f47e9c76a035d41307bf14e44c424f2.
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.