Allow registering actions per-class

Allow registering actions per-class

Add a facility to register and install actions
at class init time. The intended use for these
actions is for

a) context and other model-based menus
b) key bindings

Most of these actions are going to be stateless,
so add separate apis for the simple and stateful
cases.

We avoid creating an action group for these by
teaching the action muxer about these actions.
The action muxer also maintains the enabled
state for these actions.