Do not display caret when we're not ready

Do not display caret when we're not ready

Using a dialog may trigger a redraw at times where the metrics caches
have not been updated yet. To fix this, do as in paintEvent() and
abort caret blinking if there is an open undo group.

Move the decision to skip the caret painting to showCaret(), closer to
where real work happens. hideCaret () is now an alias for
showCaret(false), which allows some code refactoring.

See also commit c7496a11b2.

Fixes bug #11763 (although it does not trigger on master).