I18n: fix/enhance how predefined contexts are handled in py code. Now they should be specified by an attribute with the same name as the one in bpy.app.translations.contexts named tuple (i18n_contexts.default, .id_windowmanager, etc.).

This way, i18n message extracting code is now able to get the right context!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55646 954f8c5b-7b00-dc11-b283-0030488c597c