generator: support listelement-ref

generator: support listelement-ref

Support for listelement-ref needs the follwing three changes
(in the order as they appear in the patch):

* making the current list-element accessible with the variable
  xcb_listelement which is a pointer to the list-element

* supporting lists of simple-type for sumof with a nested expression

* using the variable for resolving a listelement-ref expression

Message-ID: <1409845851-38950-4-git-send-email-chris@demorecorder.com>
Patch-Thread-Subject: [Xcb] support popcount of a list and associated xml changes
Patch-Set: PopcountList
Patch-Number: libxcb 4/4
Patch-Version: V1
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>