Replace Qt's own JSON library functions by Niels Lohmann's JSON library
in the Export JSON dialog. This was necessary because for very large
JSON objects Qt's library functions generated incomplete JSON exports.
See issue #1789.
This is a first rough implementation of QScintilla support as SQL text
editor.
It should work mostly and build fine with qmake as well as cmake. The
new code supports all the features of the old one plus adding a few
subtle improvements. The main point of this, however, is reducing the
code we have to maintain and making it easier to add new features to the
editor.
Allow building the project using qmake as discussed on commit
88f66be89e. This lacks the support for generating a dynamic version
number from the Git log though and obviously most of the nice features
of CMake.
