taskswitcher: Sort tasks by last activation

Fixes #343 (closed)

This restores the sorting of tasks by their last activation. The task model now becomes a singleton and retains state even while the effect is not active, while the filter/sort proxy model is still initialized at task switcher load.

Sidenote: Activating the task switcher seems to sometimes crash for me (even prior to this MR on master) specifically when built with kdesrc-build, and it gives backtrace to kwin compositing code, not sure if anyone has investigated this