shell-app-system: Cache GAppInfos around

This was called here just to end up emitting ::installed-changed,
which would trigger other g_app_info_get_all() calls. Cache it here
so it may be reused later on.

src/shell-app-system.h

@@ -27,4 +27,6 @@ ShellApp       *shell_app_system_lookup_desktop_wmclass       (ShellAppSystem *s
 GSList         *shell_app_system_get_running               (ShellAppSystem  *self);
 char         ***shell_app_system_search                    (const char *search_string);
 
+GList          *shell_app_system_get_installed             (ShellAppSystem  *self);
+
 #endif /* __SHELL_APP_SYSTEM_H__ */