Simple preset function for setting common node sizes based on enum instead of explicit numbers.
Most nodes use the default size now and don't need explicit function calls. Most remaining nodes can also use the preset variant instead of explicit size values, these are only needed for a few special nodes.
Thanks to Sebastian König for suggesting this and doing the monkey work of changing node definitions.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54752 954f8c5b-7b00-dc11-b283-0030488c597c