Reduce visible caret flicker when caret transitions from large to small

There are significant performance costs with calculating the caret size, so still want to avoid computing this until all ops have been performed. However, if the caret size is wildly incorrect for it's new position after an update (e.g., caret moving from beside an image to beside text), the caret will be user visible before the render occurs, and results in a large caret momentarily flashing before shrinking to an appropriate size. To prevent this flicker, we hide the caret until it is redrawn, as an absent caret is far less noticeable than an oversized one.

# Reproduction steps
1. Open the editor (http://ci.kogmbh.com/deploy/WebODF/webodf-ci-b207-g6463cd14/editor/localeditor.html)
2. Place the cursor just after the image at the top
3. Rapidly toggle the caret between this position and the next empty line using the right => left cursor keys
4. Observe that the caret momentarily appears quite large on the left hand side of the screen before resizing. This happens quite reliably on Chrome, Safari + FF on OSX.