tasklist: Include grouped windows in the overflow menu
Currently (4.17.2) only single windows are handled, hopefully correctly after 6b939da5.
Implementing this should also eliminate the allocation warnings that currently appear when the tasklist size shrinks to grouped windows:
Gtk-WARNING **: 17:01:52.083: Negative content width -1 (allocation 1, extents 1x1) while allocating gadget (node button, owner XfceArrowButton)