Notifications do not show up on top of full screen windows to avoid them showing over e.g. a presentation.
To keep you from missing anything that's been going on, keep the popup open until the full screen window goes away.
There will be at most three popups shown at once, as has always been the case, and they would then cycle through.
I chose to just keep the popups open rather than using the glowing notification icon as in the original patch for simplicity and to not introduce yet another "history" of notifications (persistent + the "history" + fullscreen missed ones).
While at it simplify the "should notification close" handling by binding all conditions to a single binding

This basically resurrects https://git.reviewboard.kde.org/r/121010/ but makes use of the new libtaskmanager that should also work on Wayland and reduces roundtrips as the task manager applet is present anyway and we're sharing the backend

• Verified that making a window full screen stops notification timer for all notifications
• Verified that minimizing that window restarts the timer
• Verified that switching virtual desktops restarts the timer if the window isnt on the new desktop
• Verified that switching activities restarts the timer if the window isnt on the new activity, stopping an activity that causes a window to get moved to the current one isn't detected but that looks like a libTM bug
• Verified that hovering a notification still stops that particular notification from disappearing
• Verified that dragging a screenshot off a notification still stops that particular notification from disappearing
• Didn't actually test multi screen but it's just using the same libtm filtering logic per screen there