Split the "Interpolation Line" theme property into three for each
interpolation mode, and use them accordingly. In the current theme
refactor, almost all theme properties of the dope sheet are getting
either removed (unused), or moved (shared). This decluttering opens up
the possibility to allow more theming, and let artists read the Dope
Sheet better.
## Avoiding Confusion
There's two "Bézier" interpolation types:
- Called "Bézier" in the interpolation type menu, and
- Called "Bézier" in the code, for the types that are labeled as
**"Easing" and "Dynamic"** in the interpolation type menu.
Since this commit is about the interpolation lines, which are **not**
drawn for the former, **this PR only covers Constant, Linear, and the
latter form of Bézier interpolation types.**
Pull Request: https://projects.blender.org/blender/blender/pulls/144255
75eaecf350