Additional information in meta-model visualization

[nmacedo]

The meta-model visualization is missing some of the information that can be extracted from the signature hierarchy, namely:

• signature multiplicities
• abstract signatures
• enum signatures
• equality signatures

I don't personally use this feature very often, but it would be easy to extend to identify those features. Namely, to produce something like the example below (each of these subsets could be "painted" differently by default).

abstract some sig A {
    r : E }
one sig A1,A2 extends A {}
sig A3 extends A {
    var s : E }
var lone sig B in A1+A2 {}
var sig C = B+A3 {}
enum E { E1, E2, E3 }