[All] Fix some inheritance/composition issues and add allOf unit tests

[cbornet]

Also adds a supportMixins property for languages that support mixins/multiple inheritance.

Fix #3629 #3544 #3474 #3636

[cbornet]

@wing328 we are now selecting the interface that has a discriminator as parent. But for models that don't have a discriminator, should we select the first interface as parent like it was done before ?
I don't have a strong opinion on this:

• on one hand, it allows to have some model inheritance without discriminator which some people may find useful.
• on the other hand, if you have several interfaces, it's a bit disturbing to have different implementation for them. But I guess most of the time, models will have only one interface so that may not be a big issue.

Eventually, we could also have some "x-extends" or "x-parent" vendor extension where users could explicitely define the interface they want to be used as parent model.

[AlexNolasco]

Wondering if addProperties should account for this issue
#3829

[cacti77]

Will this PR fix issue #3904?