Draft: Add new functions for fetching input sources

The code to get the default input sources would otherwise need
to be duplicated across control-center, gnome-shell and
gnome-initial-setup.

This request avoids some of that redundancy by consolidating the
logic into two new functions:

gnome_get_default_input_sources and
gnome_get_default_input_sources_finish

It also adds a new function gnome_input_source_is_non_latin that 
callers can use to decide whether or not to add e.g. "us" to the
list for Cyrillic users.