MythUIImage: extend the random image feature to reload if a delay is used

If the <filename> is set to a directory and the <delay> is set to a value > 0
then keep reloading a random image from the directory after <delay>
milliseconds has passed.

For example this will display a random image from the themes 'background'
directory changing the image every minute (60000ms) :-

<imagetype name="background">
    <area>0,0,1280,720</area>
    <filename>backgrounds/</filename>
    <delay>60000</delay>
</imagetype>