Listen to FD.o high contrast setting

[danirabbit]

Problem

We have a few cases where we're adjusting transparency for shell elements like the Dock and Panel based on a gsettings key. This probably isn't great practice and any apps that want to use transparent elements in their styles can't listen to it without big sandbox holes

Proposal

We should add an easy way to listen to changes in the FD.o High Contrast setting and use this instead of a custom "reduce transparency" setting. This way we're using a standard cross platform setting that's available over portals and apps have access to as well

Prior Art (Optional)

No response