More consistent Live Photo pacing

Live Photos have been taking longer than ordinary photos because their motion was added on top of the full photo duration. They now fit inside the timing I selected, so a mixed slideshow has a much more even rhythm.

Changelog

• Live Photos now use the selected Photo Duration as their complete target, keeping mixed slideshows moving at a more consistent pace.
• Photo Duration now ranges from 4 to 60 seconds.
• Live Photos now meet their still image at the intended capture moment without visibly jumping backward.
• Audio-enabled Live Photos now move into the next slideshow item without lingering on their final motion frame.
• Media changes now wait for the next photo or video frame to be ready, eliminating the occasional black flash between slideshow items.
• Slow-motion videos can now be included or excluded independently from ordinary videos in Settings.