use 'const char *' by default with RNA functions except when the value is flagged as PROP_THICK_WRAP.

Also use const char in many other parts of blenders code.

Currently this gives warnings for setting operator id, label and description since these are an exception and allocated beforehand.

source/blender/editors/mesh/editmesh.c

@@ -1853,7 +1853,7 @@ static void *getEditMesh(bContext *C)
 }
 
 /* and this is all the undo system needs to know */
-void undo_push_mesh(bContext *C, char *name)
+void undo_push_mesh(bContext *C, const char *name)
 {
 	undo_editmode_push(C, name, getEditMesh, free_undoMesh, undoMesh_to_editMesh, editMesh_to_undoMesh, NULL);
 }