Ensure the event trap has a minimum height

0px height elements are not able to receive an active selection in most
browsers. Setting a minimum height ensures the event trap is still able
to be selected when in an empty paragraph.
Merge pull request #352 from peitschie/event-trap-size

Ensure the event trap has a minimum height
Remove circular dependency between OdtDocument and OdtCursor.
OdtCursor does not depend on OdtDocument any more. In fact, it is not specific
to ODF any more so at some point it could be renamed.
Make ShadowCursor independent from OdtDocument.