Slideshow with a season theme would be more interesting

[github@…]

Around the holidays it would be fun for the slideshow to contain more holiday photos, and around birthdays it would be fun for birthday photos to be shown. I'd like to see random photos from the entire year thrown into the mix too, but with a majority of seasonal photos. The existing random slideshow can remain unchanged, with a new "seasonal" option added to the menu.

I've prepared a patch already: ​https://github.com/MythTV/mythtv/pull/89.

[Roger Siddons <rsiddons@…>]

In 1bb0b6c06e7cce57051bc98f8d936ffc45b5e660/mythtv:

Gallery: Add seasonal slideshow

Add seasonal ordering to Image Gallery.
Seasonal ordering biases a random slideshow to select images around anniversaries.
Based on a patch by Chad Parry

Refs #12335

[Chad Parry <github@…>]

In 9abee9bc051cf872d953869eaf8262c1ca081d0d/mythtv:

MythGallery: Random slideshow with a seasonal theme

This creates a new mode of slideshow, called "Seasonal," to supplement the existing sequential and random slideshows.
It shows a random slideshow but with a larger weight given to photos from the same time of year.
The feature is only enabled if EXIF support is present.

I also made some fixes to the sequence classes, so that they tracked history better and could run forever without leaking memory.

Fixes #12335

Signed-off-by: Roger Siddons <rsiddons@…>