Implement togglable event targets.

Summary: This creates an interface `IEventTogglable`.  Events can be statically bound to the togglable entities with `Bind<...>().ToTogglable<InventoryUIEntity>().ToToggle("character")`.

Test Plan: N/A, for review only.

Reviewers: Sabre070

Reviewed By: Sabre070

Maniphest Tasks: T111

Differential Revision: http://code.redpointsoftware.com.au/D91