Gtk4 port - proposed 4.alpha.1

This is a port of the venerable Zenity program to GTK4.

For the most part, I have tried to maintain command-line compatibility with zenity 3.x with one notable exception: the '--window-icon' option is gone, and replaced with a libnotify-specific '--icon' option.

The autotools toolchain has been replaced with Meson.

Some old text files dating back upwards of 15+ years have either been removed or renamed to '.pre-git'.

The documentation and manpage text have been updated for the 'C' locale to reflect the changes.

Many strings in the translations are marked 'fuzzy' and have not been updated in many years. These require attention. The screenshots in the Yelp help has also not been updated from 3.x.