properties: support system-properties

Support "system" properties, i.e., library versions, whether
language-detection, scm is supported.

Make these more generic (with display strings), so we can handle them
more automatic int mu-cmd-info.

lib/mu-indexer.cc

@@ -95,7 +95,7 @@ struct Indexer::Private {
 			   store.root_maildir(), store.path(),
 			   store.config().get<Mu::Config::Id::BatchSize>(),
 			   was_empty_,
-			   store.config().get<Mu::Config::Id::SupportNgrams>());
+			   store.config().get<Mu::Config::Id::NgramsEnabled>());
 	}
 
 	~Private() {