Make width of ToolTip as constant

#### Purpose

Now Width of ToolTip depends of content it represents. This PR makes it as constant - equals to 325px.

#### Modifications

I have removed top `Grid` of UserControl. Set `Width` of `Grid` inside of main `Border` to `325`.

#### Additional references

[MAGN-5456](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5456).

#### Reviewers

@Benglin, please, take a look.