Improve download support

[thornbill]

The following improvements are needed to the initial download implementation

• Delete transcodes on server post-download — Report to server when download stops #408
• Fetch item info
• Verify item is a video before attempting to download
• Make downloads pausable/resumable
• Add messaging on empty download screen
• Improve UI of download listing
• Save playback progress
• Add quality options for downloads
• Display download file sizes
• Display available space on the device