generator: support listelement-ref

generator: support listelement-ref

Support for listelement-ref needs the following 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

Changes for V2 of this patch:
- adapt to removal of patch "libxcb 2/6" from patchset "ListInputDevices".

Message-ID: <5444E14F.4050807@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: V2
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>