feat(diagram): Fix png and diagram creation

Image generate on save fixed
- spaces in path names get properly unescaped
- resource name will always be extracted from bpmn20 resource file
  (fixes the bug that resource renaming is not reflected in png gen)

Diagram creation is fixed
- diagram will always be opened as FileEditorInput even when freshly
  created. That avoids the problem that a diagram may be opened twice
  (freshly created and via double click)

Minor cleanup on some classes done.

Closes CAM-228, CAM-380, CAM-246