cally: Refactoring "window:create" and "window:destroy" emission code

cally: Refactoring "window:create" and "window:destroy" emission code

Previously "window:create" and "window:destroy" were emitted on
CallyUtil. Although it works, and CallyUtil already have callbacks to
stage_added/removed signals, I think that it is more tidy/clear to do
that on CallyRoot:

  * CallyRoot already has code to manage ClutterStage addition/removal

  * In fact, we can see CallyRoot as the object exposing the a11y
    information from ClutterStageManager, so it fits better here.

  * CallyUtil callbacks these signals are related to key event
    listeners (key snooper simulation). One of the main CallyUtil
    responsabilities is managing event (connecting, emitting), so I
    would prefer to not start to add/mix more functionalities here.

Ideally it would be better to emit all CallyStage methods from
CallyStage, but it is clear that "create" and "destroy" are more easy
to emit from a external object