Gradient outlines more fixes

[fsih]

Resolves

Fixes #1224
Fixes #1239
Fixes #1230

Test branch: https://fsih.github.io/scratch-gui/gradientOutlines/

Proposed Changes

1. Show solid transparent instead of a gradient when the stroke width is 0
2. When you change a transparent, solid fill or stroke to one of the gradient types, it should change the color to a black-to-white gradient (instead of transparent-to-purple)
3. Remove the "mixed" (3 circles) color from any of the swatches when switching to the rectangle or ellipse tools

Reason for Changes

1. We want the state to be "the stroke width is 0 iff the stroke color is transparent" since it can be confusing if nothing is drawing because one or the other is true
2. Design decision, half-transparent gradients feel conceptually higher ceiling
3. "mixed" is not a color you can paint with

Test Coverage

Tested at a paint huddle
Tests planned later: #1216

[adroitwhiz]

I feel like a function which retrieves color state from shapes should not modify those shapes' colors directly

[ericrosenbaum]

Reviewed interactively with @fsih and LGTM!

[fsih]

Filed another bug #1248 for more tests