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.
Implementation:
* NC_SCENE or NC_OBJECT cause scene->stats to be cleared.
* NC_INFO is sent to tag info headers for redraw.
* In UI scene.statistics() creates scene->stats if it is
NULLd, and then returns the string.
