undo: split the plugin interface

[abique]

There's two plugin interfaces which both are optional.

A super minimal implementation for the plugin is to just query the host interface, and call change_made() with a null delta. This lets the host know that a change was made, snapshot the plugin state and workout the undo/redo by using state save/load.

Then the two plugin interfaces are:

• delta: optimization
• context: user interface