Allow (un)marking PyDev folders as source folders with the PyDev right-click context menu.

When right-clicking on a folder/project in the Package Explorer, allow the "PyDev" section
of the context menu to set the selection as a source folder (or to a non-source folder).
Merge branch 'addons' of https://github.com/AndrewFerr/Pydev into AndrewFerr-addons
Merged functionality to add/remove source folder with context menu. Did changes to add icon and check if pythonPathNature is not null.