Allows processing models to be stored inside QGIS project files,
so that opening the project makes that model available.
Some models are so intrinsically linked to the logic inside
a particular project that they have no meaning (or are totally
broken) outside of that project (e.g. models which rely
on the presence of particular map layers, relations, etc)
This change allows these models to be stored inside that project,
avoid cluttering up the "global" model provider with models
which make no sense, and making it easier to distribute a single
project with these models included.
Models are stored inside projects by clicking the new "embed
in project" button in the modeler dialog toolbar. Models can be
removed from a project from the model's right click menu in the
toolbox.
