Added richtext actions (with some problems)

The Richtext actions work with two exceptions:
- The font action and font size action cannot be disabled for
entries that do not support richtext. That is very irritating for
the user, but I don't know how I could disable them.
- Changing font, font size, or a color without a selection in the
text has no effect. It would be necessary to change the
currentCharFormat in the underlying QTextControl, and I have no idea
how to do that from outside. This is _very_ anying.

Apart from that a crash was fixed that could occur when all entries
are removed from the worksheet.