MAGN-4662: Tooltips for operators are 'sticky'

#### Purpose

For Operators, some Built-In functions and other functions position of ToolTip don't changes when mouse is move from top to bottom of members list.

#### Modifications

Position of ToolTip is defined in `PlacementCallback` method. Sometimes this method is not called. There was added line
```C#
this.HorizontalOffset++;
```
This line causes reposition of Popup and as result callback is executed.

Some information can be found [here](http://stackoverflow.com/questions/1600218/how-can-i-move-a-wpf-popup-when-its-anchor-element-moves).

#### Additional references

[MAGN-4662](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4662#).

#### Reviewers

@Benglin, please take a look.