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.