xdg-activation: add focus stealing prevention when using tokens

[dkondor]

Continuation of #2527

This implements focus stealing in a straightforward way by disallowing xdg-activation tokens if the view that generated them is not focused anymore. So this is about the case when the user switches to another view before a token is used.

[ammen99]

I am wondering how does this compare to https://github.com/WayfireWM/wayfire-plugins-extra/blob/master/src/focus-steal-prevent.cpp ?

[dkondor]

This works based on xdg-activation and only tracks which view has focus. Mainly, it is to prevent the following scenario:

1. User interacts with app A, which generates a token and passes it to app B
2. App B is slow to use its token, the user switches to app C in the meantime
3. Now app B uses the token to activate itself, which might be unexpected

In contrast, the plugin in wpe focuses on keyboard (if I understand correctly), to prevent taking focus while typing, regardless of whether the requesting app has a valid token or not. So I think these can complement each other, with this PR more focusing on users' attention and also reducing the cases that even trigger the fsp plugin in wpe.

[dkondor]

Marking it a draft, since I this prevents focusing also in some valid cases, especially when used in combination with #2627 and desktop components, specifically when launching apps / actions from a menu opened from a layer-shell panel (such as with WayfireWM/wf-shell#293)