Gradient outlines part 1

[adroitwhiz]

Resolves

A step towards #540

Proposed Changes

This PR implements the "refactoring" half of the "gradient outlines" changes--this should contain no functional changes but cleans up the codebase in preparation for adding gradient outline support.

Reason for Changes

See above

Test Coverage

Tests have been updated accordingly

[fsih]

Thanks for splitting all this out, it helps a lot!

I ran into an issue where if you select a shape with a hole, and set the fill to a gradient, then the fill becomes transparent:

[fsih]

Here's the same SVG exported from scratch.mit.edu vs exported from the gradient-outlines branch: https://www.diffchecker.com/xGHcgGmX

[fsih]

LGTM!

[fsih]

Can we document the params of the return value, and note that gradientType may be null?

[adroitwhiz]

I've changed applyGradientTypeToSelection to call createGradientObject when creating gradients-- this is important in part 2 when we change createGradientObject to enforce a minimum distance between the gradient start and end points (this fixes perfectly vertical gradients on perfectly horizontal lines, or vice versa, from disappearing).

[fsih]

LG!