xcbgen: new expr-type listelement-ref

Add parser-support for the new expression-type "listelement-ref"
which represents the current list-element when used inside
a list-iteration expression such as <sumof>.

This patch includes computation of the flag "contains_listelement_ref"
which is set to True when an expression or any of its
subexpressions is a listelement-ref.
(This is needed by the generator)

Message-ID: <1409845742-38797-2-git-send-email-chris@demorecorder.com>
Patch-Thread-Subject: [Xcb] support popcount of a list and associated xml changes
Patch-Set: PopcountList
Patch-Number: proto 2/8
Patch-Version: V1
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>