Temp files and folders in ".bpmn2" are not deleted when editor is closed

The editor creates temporary files which behave as FileEditorInputs, in a folder called ".bpmn2" in the project root. Since the folder name starts with ".", these are by default only visible in the Navigator view. These are placeholders for the actual *.bpmn and *.bpmn2 files that are imported during editor initialization and should be deleted when the editor is closed. Also folders and subfolders should be deleted when they are empty.

There is a side-effect that was causing two instances of the editor to collide with each other when two different files with the same name, but different extensions were opened (e.g. test.bpmn and test.bpmn2). Because the temporary folder was hard-coded as ".bpmn2", the temporary file pathnames were identical.