Displays the Nova iconography when `browser.nova.enabled` is true.
Updates 72 existing toolkit icons with the following approach:
1. wraps the existing <path>(s) in a `<g class="proton">`
2. appends the Nova redrawn icon's <path>(s) in a `<g class="nova">`
3. adds a <style> block with rules to show only one group at a time
This is an unorthodox approach. It has several drawbacks:
1. increases individual SVG size
2. increases the time required to render each SVG
3. viewing the SVGs in normal image tools will only show the Proton variant, which is confusing
However, I think this approach is better than alternatives.
1. do not need to add new Nova icon files and CSS rules to use them conditionally
2. SVG uses in XHTML and similar contexts doesn't require special handling
3. this can be undone very easily after Nova is released, leaving only the Nova paths
4. the SVG rendering performance impact is not large, and alternatives would still have non-zero performance impact
5. total size of SVGs is not that much larger than if we added separate Nova icons
Differential Revision: https://phabricator.services.mozilla.com/D301960
Stephen Thompson
0a19a7c438