Remove HDT-it! dependency on Qt 4

[donpellegrino]

The HDT-it! GUI "Compiling" documentation lists Qt 4 as a dependency (https://github.com/rdfhdt/hdt-cpp/blob/develop/hdt-it/README.md#compiling). Qt 4 is a difficult dependency to resolve as it is incompatible with modern compilers. A forum post notes problems with GNU GCC (g++) (https://www.qtcentre.org/threads/70951-error-while-installing-qt4-on-kali-linux-2020-2). I failed to compile Qt 4 with at least the following:

• Intel 2021.3.0
• g++ (GCC) 11.2.0
• Cray clang version 12.0.3

The incompatibilities were numerous across many failed attempts included passing flags to Qt 4's ./compile to turn off as many Qt plugins and features as possible.

One implementation approach might be to update the HDT-it! codebase to use a more recent version of Qt.

[donpellegrino]

I found that I was able to compile Qt 5 branch "5.12" from https://code.qt.io/qt/qt5.git. I was then able to compile the "develop" branch from https://github.com/rdfhdt/hdt-cpp.git. I did have to make some changes which I will put together in a pull request, but they were minor and did not appear to depend on any significant Qt 4 vs Qt 5 differences. However, I did observe some run-time inconsistencies in the application of subject, predicate, and object filters not applying to the list views consistently. Those might be separate issues to file.