Add search type option to search popover

A radio button group was addded to select between "Exact"
and "Substring" search types.The default search type is
"Substring" search.

We hide the search type radio button group if "All Available Fields"
is selected as exact search doesn't make sense in that case.

The GlJournalModel class was modified to handle the case when all
search fields are exact and there are no substring matches.It was also
modified to handle the case when the exact search match string has a
null or empty value in which case we do not pass it to
gl_journal_set_matches().

GL_QUERY_* prefix was added to the enum nicks for making
it consistent with the enum name GlQuerySearchType.

https://bugzilla.gnome.org/show_bug.cgi?id=767996