#537 (un)mark source folders from pkg explorer menu

[YoHell]

I'm lazy and do not want to click 8 times at different things in different windows to add/remove source directories in my project. Also, the proper place where one should do this has always been the teensiest bit hard to find, and (pardon my senility) getting harder to find in every release due to getting padded in among more and more options.

It would be neat to have an option to do this in the package explorer context menu, for example:

Package explorer menu > Pydev > Set (Unset) as source directory.

This option could apply not only to dirs, but also to containing dirs for clicked files.

Great work by the way Fabio! Pydev just keeps getting better and better with every release.