Added a read-only dimensions property for Nodes. This returns the actual node size as calculated in the node_update functions. It can be useful for node layout scripts, since the width/height properties
are not an accurate representation of the actual node size which is determined by the uiLayout.

Please note that the dimensions calculation depends on the drawing of nodes, so it may not get updated if nodes are not visible in any editor. Also the node height in particular can change dramatically
based on previews, visible sockets, etc.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57451 954f8c5b-7b00-dc11-b283-0030488c597c