Make bonds and pseudobonds selectable

[Tom Goddard]

Currently bonds and pseudobonds are not selectable although they do show a green outline if their atom endpoints are selected.

Want to be able to select bonds and pseudobonds without selecting the end-point atoms.

[Tom Goddard]

Added bond and pseudobond selected attributes in C++ and to Python API.
Added bonds and pseudobonds to Objects class and made atomspec parsing use these.
Fixed commands (color, show, hide, style, transparency, setattr, select) that act on bonds and pseudobonds.
Made selectors (hbonds, pbonds, sel) select bonds and pseudobonds instead of atoms.
Fixed mouse picking of bonds and pseudobonds.

A few things not yet done:
Save bond and pseudobond selection state in sessions.
Make drag select get bonds and pseudobonds (maybe just check bond mid-point for simplicity).

I probably introduced a number of bugs with these changes.

[Tom Goddard]

Eric has added session saving of bond and pseudobond selection.
I added drag select of bonds and pseudobonds using their midpoints.
Fixed several bugs (undo not remembering bonds, pseudobonds).