TabBox: Allow switchers to show an exit transition

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 its 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.