Move label provider extension point id string to interface
[Image Export] Add support for image export providers and improve Image support

* Export As Image Wizard
* Image Provider Extension Point
* DiagramUtils Image support improved
* More
[Image Export] Add Help context id
[Image Export] Add Export as SVG plug-in
[Image Export] Add Unit Tests
Merge branch 'image_export'
Change version number to 2.7.0

* Except for JDOM which is 2.0.5