Fix flicker when showing the status of a background fetch

This was recently introduced, but it was done the wrong way.
WithWaitingStatusSync should only be called from the main thread, and it is
meant to be used for updating the bottom line while the UI is blocked. It is a
bad idea to call this from a background thread, and it results in ugly flicker
(occasionally).

Use the newly extracted WithWaitingStatusImpl instead, this is the same as
WithWaitingStatus (which is exactly what we need) but without the implicit
OnWorker, which we don't want because we are on a background thread already.

pkg/gui/background.go

@@ -76,9 +76,9 @@ func (self *BackgroundRoutineMgr) startBackgroundFetch() {
 	self.gui.waitForIntro.Wait()
 
 	fetch := func() error {
-		return self.gui.PopupHandler.WithWaitingStatusSync(self.gui.Tr.FetchingStatus, func() error {
+		return self.gui.helpers.AppStatus.WithWaitingStatusImpl(self.gui.Tr.FetchingStatus, func(gocui.Task) error {
 			return self.backgroundFetch()
-		})
+		}, nil)
 	}
 
 	// We want an immediate fetch at startup, and since goEvery starts by