[mythtv-cmyth] Clean image cache only on startup/shutdown

Cleaning the images after a given time period is erroneous.
Channel icons won't be reloaded and it might cause unneeded
cpu load.
[mythtv-cmyth] Check if preview exists before enqueue

Check if icon preview exists before enqueuing the download job.
If not the backend will send an event to update the recording when preview has been generated.
Then Event handler will force refresh.
[mythtv-cmyth] Proginfo is already filled. Do not force cmyth_proginfo_fill()

- cmyth_proginfo_fill() forces update to get details, but we have already all details. Remove this call.