Use C++ to replace GLib and friends

[TheAssassin]

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.