RFS: Update to the GPLv2+ license.

The RARFileSource license is now compatible with GPLv3.
Updated French translation.
Fix: Play > Subtitles > Styles menu entry was incorrectly enabled/disabled.

This was broken in 0615b154c1a9e6289234a5acb6c2360b1560577c.

This commit fixes #2978.
Some testing on SetAlwaysOnTop. (not working)
PPageExternalFilters: cosmetics.

* Use more meaningful functions names.
* Use correct visibility for the members variables and functions.
Try to fix a crash in RegisterHotkeys.

This commit fixes #2971.