Allow the New PyDev Project wizards to permit project creation in empty workspace folders.

Small change: instead of prohibiting non-default project locations that overlap with the
workspace, make only the workspace folder itself prohibited. Allow projects to be created
in workspace folders that lack a .project file, regardless of whether the "Default Location"
box is checked.
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).
Remove duplicate comment line.
Prevent duplicate path names in a project's PYTHONPATH when creating src folders.

Previously, if a PyDev Project source folder was created with the same name as a path already
present in the project's PYTHONPATH, the created folder's path would be added as a duplicate.
Prevent this from happening by checking for existing pathnames before adding a new one.
In New PyDev Project wizard, fix project creation bug & use recursive search for Python files.

Make the following improvements to the New PyDev Project wizard (specifically, its first page):

1. Fix a bug where it was possible to create a new project in a workspace folder with the
"use default" setting chosen in the project creation wizard, even if that folder contained
a .project file.

2. When searching for existing Python files in the project location, now recursively search
existing folders in the location for Python files too.
Added utility to split on whitespaces.
Merge pull request #48 from AndrewFerr/exsources

Allow the New PyDev Project wizards to permit project creation in empty ...
Using PythonPathHelper to check if a file is a valid .py or __init__ file (made some fixes when checking if it's a valid __init__ file).
Merge pull request #49 from AndrewFerr/aferrazz_dev

Prevent duplicate path names in a project's PYTHONPATH when creating src...
Not doing copy of Set.
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.
Fixed test which was failing due to wrong offsets on interactive console.
When adding or removing a source folder, keep the variables from the user. Also, try to use the /${PROJECT_DIR_NAME} to make the paths project-relative when possible.