MultitaskingView: Only show and push modal if it's not already shown

This fixes an issue where we would push modal again while we were already modal
which would cause us to stay modal indefinitely because we wouldn't pop
the first modal_proxy since it was 'overwritten' by the new one.

Author: leolost2605

src/Widgets/MultitaskingView/MultitaskingView.vala

@@ -236,7 +236,7 @@ namespace Gala {
         }
 
         public override void start_progress (GestureAction action) {
-            if (!opened) {
+            if (!visible) {
                 opened = true;
 
                 wm.background_group.hide ();