Bug 280

The ability to select different colors for the locations of a model is very useful for documentation 
purposes. The color would not have any meaning to UPPAAL, but the user could assign a meaning to 
different colors, e.g. bad states, idle states, etc. The same is true for edges.

The GUI is already prepared for such a feature: The model representation contains color properties for 
all elements, and the view handlers retrieve color information from the model (in fact, this is even true 
for labels - even the font of labels is retrieved from the model). All that is needed is to:

- Add serialisation and deseralisation of the color properties to the XML and UGI formats
- Add items for selecting colors to the context menu of locations and edges
- Verify that the view handlers recognize changes in the color setting (they have to register event 
handlers for these properties).

I envision that the context-menu contains say 10 or 16 predefined colors and that the menu items are 
simply boxes with the colors (rather than the name of the color). Later on we could add a menu item for 
selecting custom colors with a color dialog.