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.
I see test cases below that pass empty string to this ```name``` parameter, then it will be ```.xxx``` (with a dot in front). I don't quite understand this method now, can you update the comment so that everyone who reads this can understand it? Thanks!