Previously, the switcher item got hidden immediately after selecting a window, so it wasn't possible to show an exit transition.
Emit instead an
aboutToHide signal and let the switchers
that opt-in to handle when to hide the tabbox by just setting
visible property to
false. In the default case we handle
that signal by immediately hiding the tabbox as usual.
For symmetry, add also a new
aboutToShow signal which
simplifies setting an enter transition.
I'm not sold on the property name
automaticallyHide, but couldn't come with something better. If we could detect weather the switcher implements a handler for
aboutToHide signal, it would be kinda superfluous.