Live C++ coding: adding and testing threadsafe::UnlockedBase

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Current stream:
Back to https://github.com/CarloWood/cppgraphviz-testsuite (a testsuite for https://github.com/CarloWood/cppgraphviz). I was working on making this thread-safe, which meant replacing certain types with `threadsafe::Unlocked` types (from https://github.com/CarloWood/threadsafe) and then fixing all compile errors.

The main thing that I changed is changing `ConstItemPtr::shared_item_ptr_` from `boost::intrusive_ptr⟨Item⟩` to `Item::unlocked_type` aka `threadsafe::ConstUnlockedBase⟨Item, ItemLockingPolicy⟩` where the latter is also incrementing the ref count on `Item` but requires a mutex to be locked before the underlying Item can be accessed. This "automatically" leads to compile errors whereever locking that mutex is required for threadsafety... and that were, and still are, a LOT of places.

One of the things that has the most impact the fact that I removed `operator-⟩()` from `ItemPtrTemplate⟨⟩`. That operator previously returned the underlying `Item*` as stored by the above `boost::intrusive_ptr`, but casted back to the derived class of the read item type (derived from Item). We can't do that anymore because we need to lock a mutex now before accessing such object. This means that code like:

graph_ptr-⟩add(foo);

that was using that operator-⟩, now needs to be rewritten as

dot::GraphPtr::unlocked_type::wat{graph_ptr_.item()}-⟩add(foo);

That is, creating a temporary Write-Access-Type whose creation locks the mutex and gives access to the underlying `dot::GraphItem`.