goodbye mix: hello mix: global.sdk_icon_color_style

[nvkelso]

Per discussion in https://github.com/tangrams/refill-style/issues/74 this PR:

• Removes from color themes the earlier styles: { mapzen_icon_library: { mix: * } } logic
• Replaces color themes logic with newer, more generic global: { sdk_icon_color_style: colorized_icons }
• Renames the earlier style colorized-icons to colorized_icons to use basemap _ underbar convention in refill-icons.yaml.
• Repoints the mix in the refill-icons.yaml styles to new global: now: mapzen_icon_library: { mix: { global.sdk_icon_color_style } } was: mapzen_icon_library: { mix: { colorized-icons } }
• Adds new default global sdk_icon_color_style: colorized_icons to each color theme file
• Switch to setting some basic icon style related globals into refill-icons.yaml for standalone imports and standalone recoloring. This impacts import order of operations, which was already true.

[nvkelso]

@bcamper @sensescape @tallytalwar @matteblair I think this is ready for review now.

I've been testing it with the import examples in the draft blog post and they all checkout now.

[nvkelso]

Related PRs:

• https://github.com/tangrams/bubble-wrap/pull/258
• https://github.com/tangrams/cinnabar-style/pull/49
• https://github.com/tangrams/tron-style/pull/155
• https://github.com/tangrams/walkabout-style/pull/196
• https://github.com/tangrams/sdk-default-style/pull/2

[nvkelso]

I made changes to the related PRs per @tallytalwar's comments in https://github.com/tangrams/bubble-wrap/pull/258. I think we're all set here now :)