Disk plugin rework

[Kritarie]

Part of this work depends on the Core redesign work.

The Disk plugin is at a very alpha / experimental, I'm not at all attached to the code or design around it. Right now the view overlays the drawer, but I think it would be better for the explorer view to display as its own activity.

• custom mimetype recognition and handling (eg. display thumbnail for image / video files)
• sharing
• fully navigable
• delete