Tooltip on MainToolbar

Before this change there was no way to see full message at least not to my knowledge...
Now this SS has 2 options first is doable by this simple change in ShowMessage method:
```csharp
if (isMarkup) {
	this.TooltipMarkup = message;
} else {
	this.TooltipText = message;
}
```
Second requires this whole pull request...

* TooltipPopoverWindow.cs functionality is same as before if not setting MaxWidth.
* I'm wonder if location of new code is correct or it should be in StatusBarContextImpl class in methods ShowMessage/Update.
* I think 2/3 of MainWindow is kind of good MaxWidth of tooltip...
* Is this proper way to obtain Width of MainWindow?
* Notice that if MainWindow is resized ToolTip will stay at 2/3 of old size until new message is set should something be done about it?(It doesn't look like a bug if old tooltip size is kept)
* this.TooltipText and this.TooltipMarkup have to be set in order to invoke tooltip...

![image](https://f.cloud.github.com/assets/774791/2287120/2e838a46-9fe9-11e3-96ef-537d2fdf106b.png)