Change pychan font definitions to XML

Fixes issue #662

PyChan font definitions are now loaded from a file with the following definition:

    <fonts>
        <font name="pickbelt" type="truetype" size="13" color="255,255,255" source="content/fonts/LinLibertineB.ttf" antialias="1"/>
        <font name="default" type="truetype" size="15" color="68,42,2" source="content/fonts/LinLibertineI.ttf" antialias="1"/>
        <font name="default_bold" type="truetype" size="15" color="68,42,2" source="content/fonts/LinLibertineBI.ttf" antialias="1"/>
    </fonts>

Fields from the old definition format are replaced one-to-one by attributes in the new format.