Support keyboard navigation over multiple monitors in multitasking view

[leolost2605]

Instead of manually handling keyboard focus where needed, do it like GTK does it with some adaptations.

We have the FocusController which is added to the "root" focusable of a focus tree (e.g. mtv or window overview).

Then we have the focusable class which is subclassed by the actors in the actor tree where focus is needed. The focusable handles deciding which implementor gets the focus based on the direction.

We now also hide the focus indicator after 5 seconds.

Fixes #2651
Fixes #2648

We can easily add navigation via Tab now too but I would leave that to a follow up

[leolost2605]

@lenemter I've added two commits on top that rename focusable to widget and then actortarget to widget target. Let me know what you think!

[lenemter]

@leolost2605 I think it would be better to leave ActorTarget as is, rename Focusable to Widget, and make Widget subclass ActorTarget. This way Widget uses gesture infrastructure, not the other way around. What do you think?

[leolost2605]

For some reason when I hold down left/right arrows in with window focused and then release it, the left/right continues to be repeated

@lenemter Hmm I can't reproduce this but maybe I misunderstood. What I did is with two windows open in the multitaskingview hold arrow key right -> the right most window gets selected -> the bell plays on repeat because the key is held but we can't navigate further to the right -> as soon as I release the key the bell stops

We should check if actor is Widget and use property to store the data, since we don't use qdata anywhere else

Now that we have the widget i've even been thinking whether we should drop focuscontroller and introduce a Root subclass of widget which would handle the focus controller stuff and would then have the focus_visible property. That's how GTK does it with Gtk.Window. What do you think?

[leolost2605]

@lenemter I updated it with a Root now