add drag and drop support of resources to PlantUML view

[anb0s]

Related to #21: sometimes during writing PlantUML comment in one component i want to see a diagram from other component without opening the related resource in an editor first, because then i have to do multiple steps: open the other component resource in editor, create new PlantUML view, wait for generating diagram and pin the view to it.

The new workflow would be:

• open/create a view and work with actual diagram
• create new PlantUML view
• drag and drop the resource from Project Explorer to new PlantUML view
  • in this case the view should generate the diagram and pin the view to the dropped resource
  • or report error if no @startuml command found etc.

[hallvard]

I've already created a new view that supports drag'n drop like this, that does not track the currently active editor. Perhaps I can combine that in a sensible way, so I don't need two views. This view supports dragging a whole project or package, which does not make sense to pin. In that case I can deactivate linking to the editor (the double arrow that previously was a traffic light).

[anb0s]

OK, it sounds good, but it may make it more heavy then and leads to more complicated implementation...