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.

Message-ID: <1409743080-302-4-git-send-email-chris@demorecorder.com>
Patch-Thread-Subject: [Xcb] xinput: make ListInputDevices work, sumof with nested expr, ...
Patch-Set: ListInputDevices
Patch-Number: proto 4/7
Patch-Version: V1
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>