Spinner to show loading tasks from Cloud

## Problem

On clicking a task list (synced to a Google Account), the application looks as if it has stopped responding. Instead a spinner would be better from the User Experience point-of-view.

## Proposal

Adding a spinner on clicking a task list or using a [Granite OverlayBar](https://valadoc.org/granite/Granite.Widgets.OverlayBar.html) as used in `pantheon-files` would be better

https://user-images.githubusercontent.com/47198395/120055689-638efb00-c055-11eb-9fc9-13400be9db38.mp4

## Prior Art

Spinners are used by other applications like Google Calendar, iCloud Reminders, etc.