CMenu: Prefer emitting events over props as functions

[codebender828]

Is your feature request related to a problem? Please describe.
The CMenu component currently accepts the onOpen and onClose function props.

However this goes against Vue's natural intuition and canbe better handled by emitting the @close and @open events to the parent context.

Describe the solution you'd like
A clear and concise description of what you want to happen.

Describe alternatives you've considered

• Change onClose prop to $emit('close')
• Change onOpen prop to $emit('open')
• Implement v-model for MenuGroup items.
• Implement v-model for MenuList items with only MenuItems children
  to it's rendering performance have to be made in the CPopover component