"Download" menu button on image detail page.

Saves original file into the SD card "Downloads" directory via
Android's DownloadManager service.

On Gingerbread the notification vanishes after downloda, but we
force the gallery to reindex the downloads area so you can still
manually bump over to Gallery.

On ICS/JB the notification hangs around and can be used to open
the saved file directly.

Note that on Gingerbread we have to download over HTTP because
DownloadManager didn't support HTTPS.

Included copy of stock 'upload' icon, flipped it for 'download'.