Add a useful comment about why we register the * extension on a ResourceSet
[Canvas Editor] Fix crash if image is removed from Image component

* DiagramImageFigure: disposing cached image
* Don't cache images, it's wasteful
* Github issue #25
[AbstractDiagramModelObjectFigure] Better tooltip logic
[ElementUIFactory] Add DiagramImageUIProvider for support of DiagramModelImage

* This paves the way for support of images in ArchiMate & Sketch models
* Also needed for Unit Tests for DiagramImageFigure
[Unit Tests] Add more Unit Tests and Test support
[Unit Tests] Better ArchimateResourceFactoryTests
[ArchiveManager] Refactor and add new accessor method

* Re-use model's EMF Resource when saving to XML
* Refactor
* Add accessor method
[Unit Tests] Add Tests for ArchiveManager
Merge branch 'unit_tests'
[Unit Tests] Fix AsyncTestRunner not working