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/makesrna/intern/rna_image_api.c

@@ -53,7 +53,7 @@
 
 #include "MEM_guardedalloc.h"
 
-static void rna_Image_save_render(Image *image, bContext *C, ReportList *reports, char *path, Scene *scene)
+static void rna_Image_save_render(Image *image, bContext *C, ReportList *reports, const char *path, Scene *scene)
 {
 	ImBuf *ibuf;