TocWidget: clean-up the timer logic

TocWidget: clean-up the timer logic

The timer logic introduced to solve bug #7138 was not entirely reliable; in
particular it resulted in spurious updates (noticeable by the treeview
collapsing just after one opens a branch, in particular).

This commit cleans up the timer logic. I followed the original design decision
of having an immediate update followed by a delayed update. Now the updates are
appropriately compressed and done after a delay of 2s (as can be noticed with
the treeview still collapsing, unfortunately, but after a more predictable
delay...).