Add Handler.initialise(controller) and call during Backend initialisation

[GDYendell]

This will enable handlers to do some logic based on fully initialised controllers that currently needs to be done in update every time. For example, filtering a list of child attributes to create status from. By default initialise will do nothing.

Acceptance Criteria

• Handlers can implement an initialise method that will be called with a initialised controller instance before the backend enters its run loop.