ui: Use AdwBreakpoint to manage adaptive UI

AdwBreakpoint is a new utility that is part of AdwWindow and AdwApplicationWindow. It allows for arbitrary changes at specific breakpoints, meaning that we can move, show, and hide widgets normally without any undefined behavior or flickering.

AdwBreakpoint allows us to remove the GtkRevealers that we used previously, as there's only a simple crossfade transition between states now.