cache: reimplemented auto-caching

Implemented a smarter auto-cache that's context sensitive. Caching will
automatically pause when playback begins or values are being changed and resume
when inactive.

app/widget/taskview/taskview.cpp

@@ -47,6 +47,7 @@ void TaskView::AddTask(Task *t)
 {
   // Create TaskViewItem (UI representation of a Task) and connect it
   TaskViewItem* item = new TaskViewItem(t);
+  connect(item, &TaskViewItem::TaskCancelled, this, &TaskView::TaskCancelled);
   items_.insert(t, item);
   layout_->insertWidget(layout_->count()-1, item);
 }