qt: don't remove acrylic visual before UI thread update

fixes "sometimes acrylic disappear for a moment when clicking outside VLC window"

basically, we want this line[1] to execute before we remove acrylic visual from the visual tree but then the QML UI thread can take its own time to post an update which is not trackable, so the solution is to just wait it out, previously I used a QueuedConnection to do the same but it doesn't seem to work with certainty. This is probably the bug of #26279 (closed)

[1] https://code.videolan.org/videolan/vlc/-/blob/master/modules/gui/qt/maininterface/compositor_dcomp_uisurface.cpp#L572