Clef movement issues

[zoemcl]

There are some inconsistencies when it comes to moving clefs:

• If there is a custos on the previous clef, moving the clef with the arrow keys moves the clef down, whether you're using the up or down keys, and the clef eventually jumps off the page
• If you try to move a clef with a custos on a previous system grouped with other elements, all the elements cluster near the clef
• If there is no custos on the previous system, the clef cannot be moved
• If you try to move the clef with no custos on the previous clef grouped with other elements, an error is thrown ( cannot read pname of null)