- Fix for junior job by bholst:

Category MEDIUM:: Currently Marble only tries to fetch tiles once.
If the server doesn't reply Marble won't retry until Marble gets restarted.
Find a solution that will reschedule the tile up to 3x in case of failure.
Between the attempts Marble should wait 1 min. before the job gets rescheduled.
Relevant classes for this job are marble/src/lib/HttpDownloadManager.{cpp;h} and marble/src/lib/HttpFetchFile.{cpp;h}