Blender Git Loki

RNA: support automatically mapping string property to char pointer

The goal of this patch is to remove the boilerplate code required to get
a string property that maps to an allocated char pointer in dna.
Previously, one to to provide three callbacks, all of which are not necessary
anymore for a simple string property.

Currently, when an empty string is assigned, the `set` function will always
allocate it as well, instead of assigning `NULL` to the pointer. Some structs
might support `NULL` while others don't, so this is the safer option for now.

Differential Revision: https://developer.blender.org/D10773