Blender Git Loki

Fix T55525: "Only Keyframes from Selected Channels" option in Timeline was being ignored

I was originally going to just expose the filtering options fo the timeline
too (via popover), to provide full access to the filtering options here too.
However, investigating further, that would've caused problems when trying to
use the Next/Prev Keyframe operators in other editors (see comment in code).
For now, the simpler solution is to just sync the scene-level flag (used for
this option) back to the dopesheet settings (used for generating the summary
channel used for displaying keyframes), to buy some time to investigate more
carefully.