Now what hasn't landed yet. I've made a bit more progress on adaptive states.

For one, everything has been renamed. Last week I posted a poll of whether it should be Breakpoint or BreakPoint, and so:

- AdwAdaptiveState is now AdwBreakpoint
- AdwAdaptiveCondition -> AdwBreakpointCondition
- AdwAdaptiveBin -> AdwBreakpointBin
- :current-state -> :current-breakpoint
- add_adaptive_state() -> add_breakpoint()

This should be clearer - that's what most people call this on other platforms, even when they aren't called that in the API.