Add new sublime.window method: open_project

There is window.open_file() that can be used for opening .sublime-project files. However, it opens the file to a new tab for editing. The suggested window.open_project() would work differently. Instead of opening the file to the tab, it would act similarly if Project - Open Project... had selected from the toolbar.

Usage:

class FooCommand(sublime_plugin.WindowCommand):

    def run(self, *args, **kwargs):

        self.window.open_project("path/to/foo.sublime-project")