Draft: Adaptive tabs

Includes !740 (merged)

TODO:

• API
  • Have an action-based API for the new tab button
  • Replace hdy_tab_switcher_open/close() with a property?
  • I don't like HdyTabSwitcher:is-narrow
  • The big picture: can we do better than requiting apps to put their tab bar into a revealer etc?
• Docs
• Tests
• Finalize styling - e.g. needs-attention could use polish
• Touch context menu goes offscreen on mobile

Fixes #241