Better download UI

[contrapunctus-1]

Currently, a user has to wait until a file is downloaded to be able to do anything else - even tapping somewhere else on the screen stops the download.

Instead, I suggest -

1. Add a 'tasks'/'operations'/'downloads' entry to the side menu
2. File downloads, including progress information, should show up here.
3. In the folders menu, show an icon to indicate that a file is being downloaded.