Randomly Chosen Variable Time Delays for Slideshows

[TheDoktar]

This pull request adds some additional syntax to the -D option or --slideshow-delay option.
The new code is fully backward compatible with the old syntax and definition of the -D option.
The old syntax allowed a real number which gave the slide delay in seconds, with - to start in pauused mode and 0 meaning a perpetual delay. For example
-D 10
Would give a new slide every 10 seconds.

The syntax is extended in two way. First a comma seperated list of numbers may be used for example
-D 5,10,20
would give equal probability of delays of 5, 10 and 20 seconds. Duplicates are allowed so that
-D 5,5,5,10,20
would give 3 chances of a 5 second delay and one of 10 seconds and one of 20.

Each number may optionially include a multiplier so that the above can also be written:
-D 5x3,10,20
As before a leading - is interpreted as start in paused mode.