Improvement pack for monogame.

 Full list of changes.
-Position property to MouseState - because its useful.
-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 it useful.
-Simple code normalization - such as moving Matrix.Decompose to good place.

PS These changes will make my experience of monogame more beauty. I hope for others too.