Add checkbox menu item widget

[martinsik]

Is your feature request related to a problem? Please describe.
Four years after I created this package I started using Kivy again (some indie game development for fun) and I found out that I really need to have something like a toggle buttom menu item widget. Basically a menu item that can be checked and unchecked.

Describe the solution you'd like
I think this widget should be part of this package out of the box because next to the ContextMenuTextItem widget these are the two most common menu items that will be used by every application.

Describe alternatives you've considered
I've already implemented this widget myself so I'll make a PR soon.

[OriDevTeam]

@martinsik Any news on the PR? I was about to create the same widget