This PR makes it so when you install an extension with icon themes it
will deploy the icon theme selector filtered down to the newly-installed
icon themes.
This is similar to what we do when installing an extension with themes.
Because we can only have one picker open at a time, when installing an
extension that has _both_ themes and icon themes, the theme selector
will take precedence.
Release Notes:
- N/A
f29b33ec85