DUI: icons dynamo units

This PR includes icons for Units category.
What is interesting - Units have methods like Operators(e.g. +,%,>, etc.), but these methods are overloaded. So, logic was rewritten a little bit. So that, we can use overloaded operators.

# Reviewers
@Benglin,@vmoyseenko, please take a look.
We need some practical examples (more than one) in the comment so the reader knows what are to expect.