[x11client] Make activity handling more consistent across windows

Summary:
Typically by default newly added toplevel windows are added only to the
current activity.

Initially windows with no borders were added to all activities. This
causes problems particularly now with the newer frame extents support
leaving window behaviour quite inconsistent.

Since the time of the original code the taskbar gained control for
controlling activities allowing at least one method of changing them.
This means we can use this as the new filter.

BUG: 274931

Test Plan:
Opened gtk3-demo
Switched activities, it wasn't on the new one
Went back, altered it through the taskmanager, it worked

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27690