New extension point pydev_interpreter_provider

Add a new extension point to allow third parties to contribute to finding,
and possibly live installing, python (and jython/ipy) installations in
new ways.

This splits the pre-existing code for locating python/jython/ipy on the
path or in the win32 registry out of the AbstractInterpreterEditor class
hierarchy, making it potentially possible to reuse the code without the
Preferences UI being loaded.
Merge branch 'development' into pullrequest/pydev_interpreter_provider
Merged pull request to configure interpreters.