xcbgen: sumof with nested expression

xcbgen: sumof with nested expression

Add parser support for sumof with a nested expression.
For example:
	<sumof ref="mylist1">
		<fieldref>bar</fieldref>
	</sumof>

The nested expression is added as the "rhs"-field of the
expression.object.

Signed-off-by: Christian Linhart <chris@demorecorder.com>
Reviewed-by: Ran Benita <ran234@gmail.com>
Message-ID: <545627E1.8070302@DemoRecorder.com>
Patch-Thread-Subject: [Xcb] [PATCHSET] ListInputDevices revision 2
Patch-Set: ListInputDevices
Patch-Number: proto 4/8
Patch-Version: V1