macosx/MainWindow: added a fast-mode to show/hide the customizable buttons, since there is no need to do CoreAnimation stuff in a hidden window (close #7360)

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

(Though in practice, the only call site ignores the return value.)

In fact, the defaut values in the preferences are better than the
default value in the wizard. If the user really needs to change the
default for IPv4 multicast, (s)he can always save it in the
preferences more permantly.

And delete contributions from Götz Waschk

We do not know how to display it anyway

Ref: #6719
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Additionally, fix a leak

It did not work at all anymore.

This will be used by the radio tuner.

This also allows to set QTSound as an input slave for QTCapture to capture both ES at the same time. (close #6851)

The VLC UIs use the VLC configuration instead of variables. That way,
updated values are propagated immediately, rather than when a video
output is created.

Since the snapshot-related variables have no callbacks, they only need
to be created if and when they are overriden/set. This only occurs with
the LibVLC media player snapshot function (which remains quite broken).