Add "Random" feature to Backgrounds

[humdinger]

The "Image" drop-down menu could offer another option: "Random by folder or query". It would open a file dialog where you choose a folder or a pre-saved query which will then serve for a randomized background image source.

After choosing the folder/query, the "Image" drop-down menu will show "Random: [imagename]" and beside it a slider labeled "Interval:" where you can set: Every bootup (slider far left) and while sliding to the right increasing the interval from 1 minute to 1 month with an intelligent stepping. Cursor-left/right could be used for exact positioning of the slider.

[humdinger]

PS:
If the slider doesn't work so well, it really is quite a spread between minutes to months, a textbox and a drop-down for unity minutes/hours/days/weeks/months and maybe "every bootup" (here ignoring the textbox value) should do.