Use a more unique data key for Gtk2::Menu position callbacks
Use a more unique data key for Gtk2::Menu position callbacks

In order to make it even more unlikely for collisions to occur, use a data key
prefixed with "gtk2perl".

https://bugzilla.gnome.org/show_bug.cgi?id=613363

cpanxaoc/perl-Gtk2,cpanxaoc/perl-Gtk2,cpanxaoc/perl-Gtk2