Show variable list properties like 'CONSTANT' and 'PERSISTENT'

[RobertoRoos]

Following

• GVL uses only the first variable List #32

Functionblocks and particularly GVLs should display these list properties. They are already parsed from PLC code, just not handled yet.

Maybe it could be worthwhile to create a variable list clause?
So manually typing a function block would become:

.. plc:functionblock:: MyFunctionBlock

   .. plc:variablelist:: VAR_IN
      :var LREAL myInput1:
      :var LREAL myInput2:

   .. plc:variablelist:: VAR_OUT
      :var LREAL myOutput1:

And a GVL:

.. plc:gvl:: MyGVL

   .. plc:variablelist:: VAR_GLOBAL
      :var LREAL VAR1:
      :var LREAL VAR2:

   .. plc:variablelist:: VAR_GLOBAL
      :var LREAL VAR3: