bug#51002: 28.0.50; [feature/pgtk] Some keypad keys handled as normal keys with pgtk

[Tom Hughes]

With pgtk some keypad keys seem to be recognised as their normal
equivalents rather than the keypad versions.

Enter, insert, delete and the navigation keys all seem to work as
expected and are recognised as kp-enter etc.

The number keys (when numlock is on) and /, (, - and + and just
recognised as normal keys from the main keyboard rather than as
kp-subtract etc.

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo 
version 1.17.4)
 of 2021-10-04 built on bericote.compton.nu
Repository revision: 4c49ec7f865bdad1629d2f125f71f4e506b258f2
Repository branch: feature/pgtk
Windowing system distributor 'System Description: Fedora 34 (Thirty Four)

Configured using:
 'configure --with-pgtk --prefix=/var/tmp/emacs 'CFLAGS=-O2 -march=native''