macosx: improve the handling of broken prefs and make sure that the user is unable to re-break them without using the command-line or a text editor

This reverts commit 8bc285bd.

The original purpose for this code is fixed somewhere else today.

close #6769

In particular, V4L is (usually) an access_demux rather than an access.

Format the text. If it is identical and all parameters are unchanged,
there is no need to update - even if the format contains a $ sign.

A slave input should be used for audio, probably ALSA or Pulse.

Improve scene video filter to write pictures in a separate thread.

macosx: trust the user that s/he knows what s/he's doing when opening a VIDEO_TS folder (close #6108)

in other words, accept folders which aren't named 'VIDEO_TS'

It probably wouldn't hurt to provide helpers for mouse events.

This is similar to hotkeys. There is no need for a hand-over thread.

Thanks to Rémi for the help

If a thread can be cancelled, it can be joined.

Regression introduced in 55c6f688.

There is no matching vlc_object_alive() or vlc_object_waitpipe().

vlc_cancel() terminates the thread. There is a massive leak though.

This reverts commit a11c642f.

This reverts commit 4dc0b22b.