Adapt caret height to context in mathed.

Adapt caret height to context in mathed.

Compute a height from current font and current cell vertical
dimensions in MathData::metrics(), because this is where current font
is known.

Introduce BufferView::setCaretAscentDescent to remember this value.

This mechanism is not used for text because Cursor::current_font is
restored by undo, and the caret height would not be changed then. But
in principle it is doable.