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/window/mainwindow/mainstatusbar.cpp

@@ -62,6 +62,7 @@ void MainStatusBar::UpdateStatus()
   if (manager_->GetTaskCount() == 0) {
     clearMessage();
     bar_->setVisible(false);
+    bar_->setValue(0);
   } else {
     Task* t = manager_->GetFirstTask();