Make the Windows GTK packaging more robust

[cameronwhite]

Currently there is just a manual list of dlls to copy during the build: https://github.com/PintaProject/Pinta/blob/master/installer/windows/bundle_gtk.targets

If the dll dependencies change (e.g. as in #859) this can break the Windows installer and might not be caught by tests (e.g. the unit tests only load Cairo, etc but not libadwaita)

Ideally this could be more automated, like https://github.com/PintaProject/Pinta/blob/master/installer/macos/bundle_gtk.py