Backport HeaderActionHook and HeaderDropdownActionHooks to 1.7.
Cherry-picks 33ed256007 from master, with the one small modification

whereby we use the old-style dropdown.png instead of ▾ in the

templates. This is for consistency, since that's what 1.7 always

used for the dropdown marker.

Passes all HookTests, and introduces some new ones, which also pass. I was also able to successfully load my TogetherJS extension on 1.7 using this patch.