DUI: scroll with Shift + Mouse Wheel

# Preface
Once it was discussed [here](https://github.com/Benglin/Dynamo/pull/63), that we need to remove horizontal scrollbar, but we should scroll with using "shift + scroll wheel" to scroll horizontally. So, here we are... I added this functionality.

# Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4663](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4663) DUI: Horizontal scrollbar is not necessary
I'm wondering why do we not need ```Keyboard.IsKeyDown(Key.Left/RightShift)``` here, I'm having the impression that this method will be called whether or not shift key is pressed.