cache: improved code pathways for cache

Viewers are now more aware of each other and can pause each others' cache tasks
when one of them plays, as well as wait for other viewers' cache tasks to
finish before starting their own.

app/core.cpp

@@ -1136,9 +1136,16 @@ void Core::CacheActiveSequence(bool in_out_only)
                                     in_out_only);
 
     // Stop any current auto-cache tasks
-    ViewerWidget::StopAllBackgroundCacheTasks();
+    ViewerWidget::StopAllBackgroundCacheTasks(true);
+    ViewerWidget::SetBackgroundCacheTask(task);
 
     TaskDialog* dialog = new TaskDialog(task, tr("Caching Sequence"), main_window_);
+
+    connect(dialog,
+            &TaskDialog::TaskSucceeded,
+            this,
+            [] { ViewerWidget::SetBackgroundCacheTask(nullptr); });
+
     dialog->open();
   }
 }