Add support for Indeterminate checkbox state

[masterdany88]

Is your feature request related to a problem? Please describe. Check box can be Indeterminate. But it should be if it takes in charge more slave check-boxes (like in list or table), see https://material.angular.io/components/checkbox/examples

[masterdany88]

https://kstatic.googleusercontent.com/files/4d0c3af7eb1dd111d3584349c3feb60047de36b1fc413e53b71d9c4175358447ccc39fb5382c91832b7a1fbeb69bd7b71f52e6407dce2abedc8c2f7fdb1e284f

[masterdany88]