Flatten header bar buttons

Well this took a while.

Requires unreleased GTK at least for tests - we may need !226 (merged) first. nevermind, CI has passed. We only use Fedora for docs and abi checker, so no tests there.

• Add an AdwSplitButton to handle that pattern in a consistent way
• Add AdwButtonContent to have icon+label buttons
  • Adjust AdwViewSwitcher buttons to have the same spacing
• Add separator.spacer
• Polish the .toolbar style, move a few headerbar-specific styles there
• Apply it to header bars and action bars
  • It's now possible to have view switchers in action bars and toolbars without messing up sizing
• Add a fill to window buttons
• Make image buttons square as non-squareness was done for header bars and now it looks weird there
• Fix a few popover-in-headerbar issues

TODO:

• Tests for AdwSplitButton
• A better name for AdwButtonContent? I intended for it to be usable with just icon or just label too but that's not doable without hacks, so icon is mandatory and will show an image-missing if not set
• Docs
• A blog post to accompany it - it's a big change and a behavior break for apps