[applets/taskmanager] Add option to cycle through grouped tasks on click

This implements a fairly common feature request to cycle through grouped
tasks when clicking on the group. There are two behaviors depending on
the active currentlytask:

- If the currently active task is not among the tasks of the clicked-on
  app/grouped task, then the most-recently used task in the group is
  brought forward.
- Otherwise, the next task in the group is brought forward, ignoring the
  stacking order, and instead going by the order of the indexes in the
  tasks model. This is deliberate; cycling through all tasks according
  to stacking order does not work because the stacking order changes
  every time a task is. Not using the stacking order for this case
  preserves the cycle order to improve the user's muscle memory when
  cycling through a large number of tasks.

As a result, it becomes ixtremely simple to switch between two
recently-used windows belonging to different apps when one or both
are within groups. With this mode, it takes only a single click. And
switching between multiple tasks of the same app likewise becomes simple
since you don't have to move your cursor from the grouped task icon;
instead you just keep clicking until you find the task you want.

FEATURE: 370258
FIXED-IN: 5.20.0