Use C++ to replace GLib and friends

To make our lives significantly easier, we want to utilize the power of C++ in appimagetool. This allows us much better memory management (no more manual `free` etc. calls etc.) and will also improve the look and feel of the code base.

Most of what we use GLib, GIO etc. for can be done with C++ out of the box in a more user friendly way, so we should be able to remove those dependencies eventually.