frontend: Slideshow rework

General changes:
- Default visibility behavior from "always play" -> "pause/unpause" to save cpu/gpu utilization
- Implemented Stop/restart visibility behavior to actually to stop, not just restart and always play
- Stop frees loaded slide textures, reloaded on restart (saves memory)
- Deactivating when visibility behavior is stop/restart frees loaded slide textures, reloaded on activate
- Manual stop/pause state will not be reset on activate/deactivate (resume where you left off)
- Manual/Automatic slides setting will not be overwritten by pressing any media controls.
- Next/prev and pause/play are re-routed to restart action if media is stopped (fixes media control ui state)
- Removed throttling from next/prev actions (more responsive ui)
- Next/Prev media hotkeys work when slideshow is in automatic mode
- Next/Prev resets the automatic slide change timer (fix double changing  slides)
- Updating settings reset pause state only when automatic/manual mode is changed (ui will remain in the pause/play state it was if mode was not changed)
- Updating settings tries to continue from existing slide position if files at that position match
- Fix transition always being recreated on settings update
- Manual Stop and Restart use cut transitions instead of the set transition speed

Signaling changes:
- Refactoring
- Activate / Deactivate send media_started / media_ended signals if visibility behavior is stop/restart as does the media control actions
- Updating settings sends media_ended / media_started signal only when appropriate (previously always media_started)

Author: Dankirk