Shift+R to repeat the last action

[Keavon]

This is a bit longer-term, once we have a more rigorously defined architecture for "actions" similar to how Blender does it (where they're searchable, and probably where you can specify the parameters that affect the action using that bottom left menu that appears when the action just occurred). That's related to #741 and #384, which are sort of this issue's prerequisites.