Listen to FD.o high contrast setting

### 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_