Blender Git Loki

This commit adds optional parameters for pyfunc implementations of RNA API (i.e. callbacks, e.g. draw functions of panels, uiLists, or exec/poll/etc. of operators). Thanks to Brecht for he review!

Any parameter after the first flagged with PROP_PYFUNC_OPTIONAL will be considered as optional, hence the python code does not have to define/use it. This will ease API evolutions by avoiding to break existing code when adding non-essential new parameters.

Note: this will need some update to API doc generation, patch is ready, will have it reviewed by Campbell asap.