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/python/intern/stubs.c

@@ -29,7 +29,7 @@
 /* python, will come back */
 //void BPY_run_python_script() {}
 //void BPY_start_python() {}
-void BPY_call_importloader() {}
+void BPY_call_importloader(const char *filepath) {(void)filepath;}
 //void BPY_free_compiled_text() {}
 void BPY_pyconstraint_eval() {}
 void BPY_pyconstraint_target() {}