[Snowcap] Allow passing an output when creating layer. #390
Description
The current implementation of Layer doesn't let the user to precise which output to use, and use the currently focused output instead.
This is fine for temporary Layers used as 'popups', but it requires explicit focus management if a layer explicitly target an output (during startup, the output signal doesn't changes the focus automatically).
It would be better to have an additional layer::new_widget_on (name to be decided later) that would take an additional parameter to explicitly request the layer to be created on said output.