add XML XInclude support for keys files
This would allow us to write a macOS specific keys file which inherits the default file.
<?xml version="1.0" ?>
<keys name="macOS default" xmlns:xi="http://www.w3.org/2001/XInclude">
<!-- inherit -->
<xi:include href="inkscape.xml" />
<!-- macOS specific overwrites -->
<bind key="f" modifiers="Ctrl,Meta" action="FullScreen" display="true" />
<bind key="grave" modifiers="Meta" action="NextWindow" display="true" />
<bind key="comma" modifiers="Meta" action="DialogPreferences" display="true" />
</keys>