Make flatpak list show both apps and runtimes by default

This makes more sense and seems what people expect, as per:
https://github.com/flatpak/flatpak/issues/490
https://github.com/flatpak/flatpak/issues/505

app/flatpak-builtins-list.c

@@ -361,7 +361,10 @@ flatpak_builtin_list (int argc, char **argv, GCancellable *cancellable, GError *
     return usage_error (context, _("Too many arguments"), error);
 
   if (!opt_app && !opt_runtime)
-    opt_app = TRUE;
+    {
+      opt_app = TRUE;
+      opt_runtime = TRUE;
+    }
 
   if (!print_installed_refs (opt_app, opt_runtime,
                              opt_system,

doc/flatpak-list.xml

@@ -47,7 +47,7 @@
             options to change this.
         </para>
         <para>
-            By default this lists the installed apps, but you can
+            By default this lists both installed apps and runtime, but you can
             change this by using the --app or --runtime option.
         </para>