DUI: remove horizontal offset in tooltip

Priority: _Minor_
# Description
This is from reviewing [another pull request](https://github.com/Benglin/Dynamo/pull/89). Having LibraryToolTipPopup exposing a public data member double `HorizontalScrollOffset` so that StandardPanel can set its value while scrolling horizontally does not sound like a good idea. The LibraryToolTipPopup should always know its location since it is a Popup class, and it can always access the LibraryContainerView to get its width in `LibraryToolTipPopup.PlacementCallback` method. It should not rely on external component to tell it where it should be positioned (external component can tell it where it should be positioned vertically, but the horizontal offset should be determined through the width of LibraryContainerView.
# Preface
We cannot completely remove horizontal offset. Problem is: we use as placement target method button. And this button has its' own coordinates, which can be less then 0.
Example:
Red arrow points real begin of method button.
![image](https://cloud.githubusercontent.com/assets/8158404/4809828/eec7d3e6-5eae-11e4-9e8a-619e3add287f.png)
# Solution
But, we can use target coordinates to align tooltip.
# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4935](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4935) DUI: Remove "LibraryToolTipPopup.HorizontalScrollOffset"