Made the left/right arrow keys invert the selection. This differs from Chimera in a couple of ways:
(1) In Chimera the left arrow key undid the last selection. This is basically available in ChimeraX as Edit→Undo as long as the selection change was the last thing you did. That, along with the fact that wanting to undo the last selection changed didn't seem common enough to assign to an arrow key, persuaded us to assign it as selection inversion instead.
(2) In Chimera, using the right arrow key without shift inverted the selection only in models that had something selected, whereas with the arrow key it would invert the selection in all models. Chimera doesn't currently have support for only-in-selected-models inversion, so the key always does global inversion.
Change: https://github.com/RBVI/ChimeraX/commit/dd9b4825d7bc9487f50a614c6c5248dd5ab538af