Improvement pack for monogame.

Please review before accept or declain. Im not added new methods, only added few useful overloads to popular structs. They will not change old programs behaviour, but added useful functionality to new.

Full list of changes.
-Position property to MouseState - because its useful to get Point from this struct directly.
-Indexers to Matrix - in special cases such when using internet templates or examples from C++ its useful.
-Operators to Point - because why not ? - this is useful in many scenario. I think XNA developers just forget about it.
-Few overloads of Contains method to Rectangle - im found this useful.
-Simple code normalization - such as moving Matrix.Decompose to good place.