Slideshow for multiple folders

[AtomicLugia]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.
• I optionally donated to support the Fossify mission.

Feature description

I think it would be a great addition to have slideshows that shows pictures from more than just one folder, for example if you have a folder with photos from spring and a folder with photos from summer and you want to have both folder's photos in one slideshow. This would be a practical feature. It should also have support for random/shuffle.

Why do you want this feature?

I tried this app and it's slideshow feature was pretty solid, it even has the random option, which is something I also wanted. However, I noticed I can have slideshows of only one single folder, instead of multiple. I have multiple galleries and sometimes I want to have a slideshow with multiple galleries.

Additional information

I had a different app called Aves Gallery, it also has a slideshow function with random and many other options, as well as support for multiple folders for slideshow. However, when the slideshow is running for a long time, it starts to lag and then either freeze or crash. While the app was decent, I had to uninstall it because this bug was very annoying. So I hope I can find a better alternative to this in terms of slideshows.

[Aga-C]

Related to #216.