Add option to only save non-frame documents when in autoshelve mode, especially useful for MAFF output.

When using autoshelve all documents that are loaded (includes frames and iframes) are potentially saved.  The issue is that when using MAFF to save the pages, it will get all those resources and stuff them into the MAFF archive.  Without these changes, you potentially save a lot of unwanted iframes, css and javascript files, etc as MAFFs, when you already have those as resources for the original non-frame top level document.

This feature can be turned on by setting the "extensions.shelve.auto.no_frames" pref.  This should probably be defaulted to on in the case of MAFF.