Breakpoints

Part 5. Includes !763 (merged), !764 (merged), !765 (merged), !766 (merged)

A breakpoint-based adaptive system, heavily inspired by UWP.

TODO:

• Docs
• Tests
• Bump version on new API
• Try to figure out the GtkBuilder issue
• Port more of the demo to it
• Fix preferences window
• Add AdwSplitButton:can-shrink
• Use GtkButton:can-shrink in the demo
• Use shrink-center-last in header bars