Don't search the local disk for remote images in LoadScaleImage()
If we fail to load the image then report that we failed
Don't add an invalid (null) image to the image cache
Increase the header cache time from 5 minutes to 20 minutes. This is still less than the average web browser. I can't think of an example where this higher time, which only applies to Modification timestamp checks would cause problems but it potentially speeds up the UI on certain screens measurably.
Fix MythDownloadManger after 997f7aa10. It was possible for queue and cancel requests to be handled out of order resulting in newly queued downloads being cancelled almost immediately. We avoid this by removing any cancelled downloads  from the download queue to a new cancellation queue, then we clean up the cancelled downloads later without affecting new additions.