WIP Input focus update

[ids1024]

Not too much to see here code-wise at the moment (just an initial attempt at combining LockSurface and Popup focus targets into a WlSurface target). But I figure I may as well start a WIP PR now that I've been looking into what to change here.

Updating cosmic-comp for Smithay/smithay#1326 is fairly complicated. Particularly trying to do things more correctly, and not just copying the logic from the trait impls removed there. Adjusting how pointer focuses are defined without updating to that commit seems like a good place to start iterating on things without have to address everything at once.

The Wayland protocol spec doesn't define how exactly input to subsurfaces works, but on KDE, Gnome Shell, And Sway:

• Subsurfaces the mouse moves over get pointer focus
• Subsurfaces never get keyboard focus

On cosmic-comp, it seems with a couple subsurfaces, some but not all subsurfaces can get pointer focus? So something isn't quite right.

Given how we were implementing PointerTarget for Popup and LockSurface to just call the PointerTarget methods on the WlSurface, we're presumably also not sending pointer input to subsurfaces of the LockSurface or Popup. So we want to have a PointerFocusTarget::WlSurface with the subsurface.

Meanwhile, for KeyboardFocusTarget, we don't want to have a subsurface as the focus. And it seems some code here does care that the keyboard focus is a popup... I wonder if condensing into a KeyboardFocusTarget::WlSurface makes as much sense as the simplification of combining surface types into a PointerFocusTarget::WlSurface.

[Drakulix]

Closing in favour of #380. Thanks for starting this!