feat(diagram-handling): open non-workspace files

This commit unifies uri handling for various resources that
may be opened by the modeler as BPMN 2.0 files.

This allows us to open any kind of BPMN 2.0 file, independent of
whether it belongs to a workspace or not.

* FileService#getInputUri() is responsible for retrieving
* Equals check for BPMN 2.0 DiagramEditorInput normalizes the inputs to
  file system absolute path
* DiagramExport exports diagrams only in Workspace mode

Closes CAM-1648