Custom node icons

- Defined 5 vertex (node) types: Generic, User, Mobile, PC and AP.  Adde...d custom icons for each type / state of the vertices.

The node type can be changed manually by clicking on the vertex label and selecting 'Change Type -> ...'
- (Experimental) The type of the vertex is initially determined based on the total number of connecting edges, after the graph has been generated.
- Added an option in the 'View' menu to change the zoom mode (direction) between  Mac OS / Windows scroll styles.