Improve propTypes validation

[adamkudrna]

We would like to validate that:

• mutually exclusive props aren't set at the same time
• parent component of a multi-level component only receives children of certain type (eg. Toolbar only accepts ToolbarItem and ToolbarGroup and nothing else)
• component passed in props where a given type is required (i.e. ScrollView prop in the Modal component)

Make use of https://github.com/airbnb/prop-types ?

Relates to facebook/prop-types#327 (comment)

[bedrich-schindler]

We are closing this issue. We've decided that we will not try to validate children as we do not know whether library user will not want to use wrapped version of supported elements which is a thing we are not able to validate in simple way.