define new gdk button constants

[d47081]

https://docs.gtk.org/gdk3/const.BUTTON_PRIMARY.html https://docs.gtk.org/gdk3/const.BUTTON_MIDDLE.html https://docs.gtk.org/gdk3/const.BUTTON_SECONDARY.html

<?php

var_dump(
    Gdk::BUTTON_PRIMARY // 1
);

var_dump(
    Gdk::BUTTON_MIDDLE // 2
);

var_dump(
    Gdk::BUTTON_SECONDARY // 3
);