This patch adds snapping options for the playhead to all animation editors.
The options can be modified through a new dropdown in the editor header.
All editors will show all those options, and they are shared,
so toggling the option in on editor will change it for all other editors too.
Some options are not working/relevant in some editors for example
Strips in the Dope Sheet. However for consistency the option is still shown.
This is a separate menu from the transform snapping menu because
you can toggle the snapping for transform and playhead separately.
Putting it in the existing snapping transform menu would imply that it can
be turned off with the magnet which is not the case.
Playhead snapping is explicitly disabled for the drivers editor
because there is no playhead to drag around.
Snapping to Frame/Second intervals takes the scene start as a starting point.
That means you can snap to the n-th second of the animation even though
it might not start at frame 1. The preview range is NOT taken into account
by design since the use case is working on a sub-section of the animation
in which case the snap target should not change.
Snapping is toggled by pressing CTRL as indicated by the status bar.
Snapping to Frames/Seconds is absolute, meaning no matter
how far away your cursor it will snap to the closest snap point.
All others only snap to things if they are close to the cursor in pixel values.
When mixing those two behaviors, it prefers relative snapping.
If no point is close enough to snap relative,
it will fall back to absolute snapping.
Based on the prototype #135913
Part of #135794
Pull Request: https://projects.blender.org/blender/blender/pulls/137278
87a28fa117