Identify properties by ID and type

[ScriptRaccoon]

An issue that already became apparent while working on #242 was that, sooner or later, properties can no longer be identified solely by their ID in the database, i.e. by their name, since the same name can be used for properties of different structure types. For example:

regular category != regular functor

Thus, we cannot have a single entry with the primary key regular in the properties table (which, after #242, contains properties of all structure types). Other examples that will become problematic once additional structure types are added include:

initial functor != initial object
isomorphism (as a property of a morphism) != isomorphism (as a property of a functor)
cocomplete category != cocomplete monoidal category

To fix this, the primary key of the properties table is now the composite key (id, type). As a result, a couple of queries (though not many) had to be adjusted. (Of course, using a longer single primary key like regular_category is inconvenient.)

Remark: the structures table and the property_assignments table remain unchanged, even though one could argue that, for consistency, the type should also become part of their primary keys. This is not necessary at the moment, and if the need arises, it can be addressed in a later PR.

To demonstrate that the change works, the new functor properties "regular" and "coregular" have been added. Both properties have been decided for all functors in the database. (In fact, this motivated adding the property of preserving regular epis/monos in #262, which was merged recently.)

Other examples that may be added later: "additive functor" (can now be distinguished from "additive category"), "accessible functor" (can now be distinguished from "accessible category"), "cartesian functor", "topological functor", etc.