Add cursor-class property to stylesheet

This prevents you from having to create a new RML file for each cursor, given most will look identical with different decorators and offset.

This will simply set the specified class on the body element of the cursor document.
This class can then be targeted from within the document's styles to apply different decorators etc.

An example:
```html
<rml>
<head>
<title>cursor</title>
<style>
body
{
width: 15px;
height: 20px;
}

body .cursor
{
position: relative;
width: 15px;
height: 20px;
left: -5px;

cursor-decorator: image;
cursor-image: canvas.png 10px 16px 25px 32px;
}

body.pointer .cursor
{
cursor-decorator: image;
cursor-image: canvas.png 25px 14px 40px 34px;
}
</style>
</head>
<body>
<div class="cursor"></div>
</body>
</rml>
```

Then somewhere in a stylesheet:
```css
button
{
	cursor: "cursor";
	cursor-class: "pointer";
}
```