Allow project PYTHONPATH updating for all drag & drop operations on folders.

Provide the PydevPackageExplorer with its own drag & drop service that chooses PyDev drop
assistants over all others (instead of the other way around, which is the default). By so
doing, allow project PYTHONPATH updates for all cases of dragging & dropping folders,
including cases where a non-source folder containing source folders is moved.
Merge pull request #66 from AndrewFerr/bugfixes

Allow project PYTHONPATH updating for all drag & drop operations on fold...