FEATURE: Disable specific tree nodes in TreeDropdownField

This feature request was born out of wanting the ability to disable (for example) a top level page from being selected, while still being able to select a child page. Using `setFilterFunction()` simply removes the node and its children.

Usage example:
```php
$tree = new TreeMultiselectField('MyTree', 'My Tree', 'SiteTree');

$tree->setDisableFunction(function($obj) {
	return ($obj->ClassName == "DisableMe");
});
```

Appearance of a disabled node:
![screen shot 2013-05-30 at 16 21 36](https://f.cloud.github.com/assets/1655548/585744/29c20204-c93d-11e2-83f9-b863af07a871.jpg)

The (minor) CSS changes have been tested in Chrome, Firefox, IE8 & 9. Amended one of the graphics slightly, but thanks to tinypng it's actually quite a lot smaller in terms of filesize.