Limit caret redraws to a maximum of once per cycle

Any operation or group of operations that affected multiple paragraphs would cause the caret to re-render multiple times.

Now, updates are stored, and the final caret state is redrawn only once on the next cycle.

On my machine this produces the following improvements in the "Bold the current selection" step (all tests performed on 100page.odt):
|   | Master | PR |
|---|------------|------|
|FF|3509|2644|
|Chrome|6471|1979|