This is a request from the animators at BI.
Now you can easily assign shortcuts from "X-Axis Mirror" and other boolean settings in the UI.
However this patch needs more work as I'm unsure if this is the correct way to implement this.
I tried to assign operators to the buttons in question, but it seemed like this was not intended to happen and I ran into quite a few issues. So I opted for this simple approach instead.
Note that this patch introduces a memleak as there doesn't seem to be a easy way to free up the new shortcutData after the menu has been created.
(But I'm guessing I might be missing something obvious)