Allow (un)marking PyDev folders as source folders with the PyDev right-c...

...lick 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).

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I thought it was a good idea to add this feature. The problems with it now are 1) the "disable" button appears in a different row as the "enable" one does, because they have different objectClasses, and 2) in Package Explorers that aren't the PyDev one, the "disable" never appears.