Now toolbars stay open on mouse-out, and close on other toolbar open,
outside click or an accepted action.
To support:
- Added new system to track and manage open dropdowns.
- Added way for buttons to optionally emit events upon actions.
- Added way to listen for events.
- Used the above to control when dropdowns should hide on action, since
some dont (like overflow containers and split dropdown buttons).
Adds dynamic and fixed (out of DOM order) positioning with location
adjustment depending on space.
Also adds smarter hiding to prevent disappearing when mouse leaves but
within the same space as the toggle.
