DesktopIntegration: Take stacking order into account

[leolost2605]

We sort windows by stacking order and emit windows_changed when the stacking order changes

[leolost2605]

We now don't emit windows_changed when stacking order changes because the actual current stacking order is only needed on specific occasions by the dock where it's more efficient to just manually sync the windows instead of having signal emissions like all the time.

[danirabbit]

I think what I'm seeing with this branch is if I have 3 windows I sometimes get a case where scrolling will go like:
1 → 2 → 1 → 2 → 3

instead of just
1 → 2 → 3