Added registerable size properties for node types in RNA. This allows pynodes to define min/max and default width for nodes. Height is also registerable, but will be ignored for the standard node layout doable with pynodes atm (height is calculated automatically based on sockets and extra buttons). It might be usable in the future, for now just added for completeness.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56235 954f8c5b-7b00-dc11-b283-0030488c597c