[Lens] Improve color stop UI

[flash1293]

As discussed offline, the color stop ui in Lens should be improved. Details in the wireframe

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[ghudgins]

slight requirement change request (simplification): simplify the dropdown palette preview to show equal proportions of color since what it's doing today can be missleading--especially on single metric visualizations that don't have data domains (min / max).

This was discussed with @MichaelMarcialis and myself and I think we're in alignment...i'm comfortable losing the functionality as it stands today to bring it back in a follow-on enhancement. I'll create an issue for the enhancement including this and some points raised by @gvnmagni (like visually modifying the stops more intuitively) which will be a more value-add and elegant solutions to this problem.

[ghudgins]

Design update - switch infinity symbol to "Min" and "Max" respectively to improve understandability. CC @MichaelMarcialis & @gvnmagni

Nice to have - "Distribute values" functionality. This one needs a design proposal and may belong in its own issue depending

CC @flash1293