[mythtv] [Patch]Movie poster download feedback

[J. Donavan Stanley]

The attached patch make the timeout value for the movie poster into a 
configurable value with a default of 30 seconds.  The previous hardcoded 
5 second timeout was too short for folks on dialup to retrieve posters 
on a reliable basis.

This patch also adds a messagebox to warn the user in the event of a 
timeout retrieving the movie poster.