Bugfix SUP-1395
When doing drag&drop of blocks in tables, the table cells were also
selected when hovering over them. Also empty table cells were not marked
as possible drop target (although dropping worked).
Both issues have been fixed.
Bugfix SUP-1403
Drag & drop of blocks in tables could interfere with resizing of table cells.
This will be prevented now.
Bugfix SUP-1406
Drag & drop of blocks sometimes caused a JavaScript error,
which will be prevented now.