getMapProject and getSelectedProject for #106

* created getMapProject and getSelectedProject to retrieve the project
according to the context
* all actions use either getMapProject or getSelectedProject (manually
added to find the right context
* refactoring of possible NPEs
* removed deprecated (and error prone) WorkspaceUrlHandler.java
TODOS:
test: AddOrUpdateReferenceEntryWorkspaceAction, DocearRemoveRepositoryPathAction
fix:
* DocearController.findProject()
* WorkspaceModelExtensionWriterReader.WorkspaceModelExtensionWriterReader(...).new IMapLifeCycleListener() {...}.onCreate()
* MModeWorkspaceLinkController.createRelativeURI()
questions:
* remove "Project" related actions from ribbon's "Home" task?