Improve undo of consecutive insertions/deletions

Improve undo of consecutive insertions/deletions

The old scheme was:
 * multiple insertions are undone by groups of 20
 * multiple deletions are undone in one big block

The new scheme is to stop merging undo elements after 2 seconds of elapsed time.

Moreover, the merging of undo elements stops when the cursor has moved. Potentially, this could allow to remove many of the finishUndo() calls.

Fixes bug #9204.