Enhance hiding dropdowns

Created by: cervengoc

Enhance hiding logic of dropdowns to allow more flexible customization.

• Pass the click event which is about to hide the dropdowns to hide.bs.dropdown event parameters as relatedEvent, so developers can decide to hide or not using for example e.relatedEvent.target.
• Expose clearMenus as a static method of Dropdown so it can be overridden by developers just as prototype methods