Add source view

This pull request adds a Source tab to the modeler, allowing it to switch between design and source (áka XML) view.

Both views are editable and synchronize (with some minor limitations).
I refactored the import / refresh logic of the BPMN 2.0 editor in order to properly implement the refresh logic.

As a side effect, validation works for non-workspace diagrams now.

And save as works as expected, too.