Improved photo playback/slideshow

[pannal]

GHI (If applicable): #

Description:

Fast pre-loading photo slideshow

• preloads the current, previous and next items using temporary files
• takes care of temp file cleanup (holds temporary instances of a maximum of 10 photos and their backgrounds)
• shows a transition between on changes
• tries to not show any loading indicators if possible
• smoothly handles action spam (multiple next requests in quick succession for example)

Checklist:

• I have based this PR against the develop branch

[pannal]

This is actually not very proof-of-concept-y anymore. I'd like to add a setting for the slideshow interval and maybe a resolution setting.