Fix inheritance implementation

[sgraband]

Currently you cannot delete inheritance edges, by clicking on them and pressing ENTF/DEL. For the other edge types it is working.

Furthermore, inheritance edges cannot be moved, since there is no representation in the notation files.

[sgraband]

@CamilleLetavernier should this be added to Milestone 1? Currently you cannot delete inheritance edges at all, and therefore causes the tests to fail.

[CamilleLetavernier]

Good point, this definitely makes sense

[sgraband]

Can you assign this to me? I found some other problems with inheritance that should be fixed, before working on the tests.