Drag&Drop-Support for entries.

Currently only CommandEntries can be draged, but that is only because
dragging an entry is currently started by dragging its prompt. Once I
know how other entries should be draged they should work as well.
Stuff that should be fixed includes:

• When the entry being dragged has a LaTeX-result this is currently not visible in the drag-cursor.
• Resizing the placeholder entries that indicate possible drop-spaces currently happens independently from other spaces, so that dragging an entry around can lead to temporary increases in the worksheet size. This is ugly, but it easy to implement with the current design.