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:

### 100pages.odt - Bold the current selection
|   | Master (ms) | PR 321 (ms) |
|---|------------|------|
|FF|3509|2644|
|Chrome|6471|1979|

### 1000page.odt - Bold the current selection
|   | Master (ms) | PR 321 (ms) |
|---|------------|------|
|FF|-|-|
|Chrome|416651|42446|