Instead of requiring a const char* return from the (optional) node label callback function, let it write into a mutable string buffer. This will allow actual dynamic labels for nodes using the python
API.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61236 954f8c5b-7b00-dc11-b283-0030488c597c
Make dynamic node labels possible as a registerable function 'draw_label' (simple 'label' identifier is already in use, need to avoid API breakage). This should simply return a string. The dynamic label can still be overridden by the user-defined node.label string.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61237 954f8c5b-7b00-dc11-b283-0030488c597c
Small example for dynamic node labels in the custom_nodes.py template script.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61238 954f8c5b-7b00-dc11-b283-0030488c597c