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