Suggestion: add color ramp options

[frasercl]

Use Case

As a user, I want to pick a (potentially preset) color gradient for a channel transfer function without manually colorizing control points in "advanced" mode.

Solution

This is the one idea in Dan's transfer function editor idea issue #131 not included in the redone transfer function editor work in #294.

Potentially add a dropdown to basic mode with some preset gradients?

Alternatives

Decide not to do this? Add the ability to color pick on basic mode min/max sliders, as a user currently can on advanced mode control points?

[toloudis]

Just in case there is something to be learned (maybe not), also see
allen-cell-animated/agave#207
and the work in progress PR:
allen-cell-animated/agave#155