First pass at submenu support

[mdo]

Adds submenu support to dropdowns. Needs a big cleanup.

To fix the problem, remove the unused constant so that the code only defines event constants that are actually used. This eliminates the dead variable and any confusion about whether a mouseenter data-API event is supported.

Concretely, in js/src/dropdown.js, delete the line that defines EVENT_MOUSEENTER_DATA_API. Leave the other constants (EVENT_HIDE, EVENT_HIDDEN, EVENT_SHOW, EVENT_SHOWN, EVENT_CLICK_DATA_API, EVENT_KEYDOWN_DATA_API, EVENT_KEYUP_DATA_API, EVENT_MOUSELEAVE_DATA_API) untouched. No new imports, methods, or additional definitions are needed.

In general, the way to fix an unused-constant problem is either to remove the unused declaration or to start using it where it was intended. Since we have no evidence of a missing mouseleave handler elsewhere in the shown snippet, the safest fix that does not alter existing behavior is to remove the unused constant.

Concretely, in js/src/dropdown.js we should delete the line that defines EVENT_MOUSELEAVE_DATA_API. All surrounding constants (EVENT_HIDE, EVENT_HIDDEN, EVENT_SHOW, EVENT_SHOWN, EVENT_CLICK_DATA_API, EVENT_KEYDOWN_DATA_API, EVENT_KEYUP_DATA_API, EVENT_MOUSEENTER_DATA_API) remain unchanged. No additional methods, imports, or definitions are required; we are simply removing dead code.