In the New Python Project wizards, enable linkage to existing source folders.

In the New Python Project wizard for PyDev, Django, and Google App Engine projects,
add an extra page to allow selection of existing external source folders to include
in the project.
Allow adding existing source folders to PyDev projects.

"Link to Existing Source" is added to the types of new resources that can be created
for an existing PyDev project.
Add a "Create links to existing sources" option in New PyDev Project wizards.

Instead of always having the option to add existing sources, this new option must be selected
in order to permit the selection of existing sources to add.
New PyDev Project UI notifies the user if project location contains existing files.

If the project directory is set as a location that includes Python files, a message
appears at the top of the wizard window to notify the user of this. A special message
appears if the location contains a file named "__init__.py".

Also, non-default project locations that overlap with the workspace are prohibited.
Merge branch 'exsources' of https://github.com/AndrewFerr/Pydev into AndrewFerr-exsources