change/add pseudobond selection cascade

[Elaine Meng]

Now that pseudobonds can be selected separately from their endpoint atoms, it would be very useful if a pbond selection could be broadened to include its endpoint atoms. This is already the case for bonds, that is, "select up" or keyboard up arrow broadens a selected bond to the bond plus its endpoint atoms.

Currently a selected pbond broadens to the pbond model.

Proposed change is making alt-up arrow (command "select alt-up") broaden from pbond to pbond model, and up arrow alone (command "select up") broaden from pbond to pbond+atoms.