applets/taskmanager: Fix cycling through tasks with a fullscreen window

When there is a fullscreen window of some program, the task manager always focuses the fullscreen window first instead of the last used window as expected.

To reproduce :

• With multiple displays, with task grouping and "cycle through tasks",
• Open multiple windows of app 1, make one of them full screen
  • For instance, a browser with a fullscreen video on one screen and other browser windows on the other screen
• Switch to one of the non-fullscreen windows of app 1
• Switch to another app 2
• Come back to app 1 by clicking the group in the task manager

-> The fullscreen window gets focused instead of the last used one

This fixes it by using the last activation time instead of the stacking order when returning to the last used window in the "cycle through tasks" mode