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