plus some cleanup

Hi there

my VLC 2.0.1 has been crashing randomly on OS X 10.7 during Time Machine backups, and it was a thread-safety issue.
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

We should not use shift, because this triggers the slow motion maximize animation feature,
at least on lion.

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

Close #6489
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

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

Close #6481
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Pointed-out-by: Rémi Duraffort <ivoire@videolan.org>

add_callback could not work correctly with the (lack of) Lua threading
support. Fortunately, it was not in use.

del_callback depends on add_callback.

Fixes combo visibility in open_panel

Visibility changes were not applied to the created qwidget
except for base class.
Changes were also not implemented for some classes.

vout_macosx: check whether the receiving window can respond to non-standard calls prior to calling them (close #6034)

If the interface script has nothing to do, then it can simply end and
the let the interface thread finish ahead of pf_deactivate.
This will _not_ automatically cause VLC to exit.

If there is nothing to do, then the interface thread can finish.

The Lua interpreter is not thread-safe. So this API made absolutely no
sense: the timer callback is called asynchronously from another thread.
Fortunately, none of our Lua scripts used this except for testing.

(This should probably be moved to Lua code.)

Pointed-out-by: Edward Wang <edward.c.wang@compdigitec.com>