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'll try to write clear comment, but first I'll show what happens, if we don't add HorizontalOffset.
![image](https://cloud.githubusercontent.com/assets/8158404/4471857/6cd86a1e-493f-11e4-9619-0802f228e57f.png)