Demonstrate how to use text snippets

[mgarstenauer]

This PR adds an Insert Snippet button that inserts a text snippet at the caret position.

The snippets insert the text

public class Name
{
    public Name()
    {
        |
    }
}

where Name is a replaceable text part. When the class name is changed, the constructor name is automatically updated. When done, the caret is placed inside the constructor body.

This should help newcomers to get started with text snippets.