Don't allow "same-type" linear container children creation

[abey79]

Nested linear containers are currently auto-merged, since it doesn't make any sense to have a, e.g., horizontal container directly inside another horizontal container. From a viewport perspective, this is strictly equivalent to a single horizontal container with all the children.

horizontal
└ A
└ horizontal
   └ B
   └ C

same as:

horizontal
└ A
└ B
└ C

This means that creating a Horizontal/Vertical container inside of a container of the same type, e.g. with the "Add SV/Container" modal, is a noop that is surprising to the user:

==> The modal should disable "same-type-linear" container with a tooltip explaining why.