New port net-im/nheko

Third time's the charm for Qt-based Matrix clients; this one looks pretty
good and seems to work pretty well. It also offers registration for new
users, and understands both plain (matrix.org) and premium (modular.im)
accounts.

The port contains the following yuckiness:

• a hack to change <json.hpp> to <nlohmann/json.hpp>, since the CMake bits in nheko don't actually go look for it.
• a (GH_TUPLE) copy of lmdb++, which is #included by the C++ code, but CMake doesn't go looking for it.
• a (GH_TUPLE) copy of tweeny, which is used by the C++ code, but CMake doesn't go looking for it. Since this is a source copy, the header isn't in the tweeny/ directory (where it gets installed), so an extra hack is called for to provide that subdirectory.

I considered making tweeny a separate port (many Linuxen do), but
this would be the only consumer, and it doesn't even consume the whole
library but only parts of one header.