Shift+R to repeat the last action

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.

No files, tests, or entry points are identified. Start by reading the action-architecture prerequisites in issues #741 and #384; done means Shift+R can repeat the last action within the rigorously defined action system.

Written by the indexing model from the issue text.