Locked `FCurves` had 2 visual issues
* the dashing is so short it just creates visual noise
* keyframes are drawn in white looking like they are selected
This PR changes this by
* Increasing the dash width
* Keyframes are drawn in black on locked curves
* To indicate that they can't be selected, draw them in a X shape
* To further reduce visual noise, locked curves no longer draw thicker when selected
This is part of the changes discussed in this design task #104867
Pull Request: https://projects.blender.org/blender/blender/pulls/106052
6332d1b8a8