Restore QML plugin(s)

[jpnurmi]

In order to achieve maximal compatibility, they must NOT introduce new wrapper types of any sort, but merely register the existing C++ types.

• https://github.com/communi/libcommuni/blob/master/examples/quick/main.cpp
• https://github.com/communi/libcommuni/blob/1.3/src/plugins/quick2/plugin.cpp
• https://github.com/communi/libcommuni/blob/1.3/src/plugins/declarative/plugin.cpp