Can successfully parse the following XML and and implement the action:
<mouse>
<context name="TitleBar">
<mousebind button="Left" action="DoubleClick">
<action name="ToggleMaximize"/>
</mousebind>
</context>
</mouse>
The XML parsing code for this looks A LOT different than the already
existing XML parsing code. It may have to be reworked
title_height is a derived variable which needs both config and theme
variables (font height and title padding). The code is tidier calling
post_processing() for this from theme_init()
