Rewrite the code that sets caret height in mathed

Rewrite the code that sets caret height in mathed

Instead of remembering the caret ascent and descent for the cell that contains the cursor, the new code remembers caret dimension for each MathRow object. This makes the code much less fragile (ans slightly smaller).

This fixes caret size issues when the Update::SinglePar flag is active.

Fixes bug #11541.