Support Marking Operator Properties as Basic or Advanced

With this it's possible to split operator properties into basic and advanced ones. This will be needed so top-bar can move advanced properties into a "More" popup. We agreed on supporting this during the 2.8 UI workshop (see https://wiki.blender.org/index.php/Dev:2.8/UI/Workshop_Writeup#Top_Bar)

How it works

The current implementation is a simple solution, but it's rather evil :)

All properties added after a call to WM_operatortype_props_advanced() are considered advanced.

Internally, this does nothing but counting and storing the items added so far, which gives a threshold between basic and advanced props. To draw properties separated in the UI code, I added WM_operatortype_is_prop_advanced (runs a loop over properties currently, shouldn't be a performance issue).

I'd mainly like to hear if this approach is fine, or if it's too ugly. If it's fine, I'll probably just remove testing code in interface_layout.c and commit to master.

For Testing

For testing, I made the redo panel draw only basic properties. And I also slit up draw-able properties into basic/advanced of a bunch of operators:

• All transform operators in transform_ops.c
• All VIEW3D_OT_snap operators (only one had properties)
• Some mesh-edit operators (loop-cut, subdivide, mark seam, mark sharp, smooth vertex, laplacian Smooth vertex, merge)

Open Questions

• How would this work for Python defined operator properties?
• How should calling WM_operatortype_props_advanced() multiple times be handled? (E.g. calling once in TRANSFORM_OT_translate and once in Transform_Properties. Currently all properties added prior to the last call are considered basic)
• How will we update operators to use this? Updating all operators would be a tedious task.
• How should UI handly operators that only have advanced properties? A "More" button would be weird then. (More of a top-bar design question)