Add g_settings_schema_key_get_summary/description

Add an API to read the summary and description from the .xml schema
files.

This will be used by dconf-editor and gnome-tweak-tool.

This API is a bit heavy -- it parses the XML and builds a table.  It
also loads gettext domains for translation.  It only does these things
if it is used, however, so it will not impact normal applications.

We store the summary/description in a pair of hash tables on the schema
source (which we have a backref to as of a few commits ago).  We can't
use a global table because people might want to request summary and
description from non-default sources.  We don't want to use per-schema
tables because we'd have to reparse the directory every time (since we
cannot guess which file a schema may have been in).

https://bugzilla.gnome.org/show_bug.cgi?id=668232