label: Add support for spans with classes
Building on the work of links in labels, this adds support for adding spans to labels with the form:
<span class="class-name">some text</span>
In an effort to be compatible with HTML, this also supports nested spans and css nodes. To be backwards-compatible, if the span contains pango-specific attributes, then only the class attribute is removed from the markup, and the remaining attributes are passed down to pango.