bug#60669: Linux Emacs 27.1 custom toolbar icons

[David McCracken <davidm@ixont.com>]

The Emacs 27.1 provided (apt-get install) by Ubuntu 22.04 reduces 
support for custom toolbar icons. It only works with B/W pbm files and 
only if located in /usr/share/emacs/27.1/etc/images and only if there 
isn't an xpm file with the same base name. i.e. name.pbm works but not 
if there is a name.xpm in the directory. If these conditions are not met 
nothing is displayed.

This does not seem to be baked into Emacs 27.1 because these 
restrictions don't exist in Windows Emacs 27.1. They are also not 
inherent in Linux because they don't exist in Linux Emacs 26.1.

There are two serious problems with these seemingly unnecessary changes. 
B/W icons reduce not just the aesthetic quality but also subtle visual 
clues that help an icon convey its meaning. Accepting only icons in the 
version-numbered installation directory means that custom icons are 
wiped out by upgrading Emacs.

The installer program I provide with my Emacs library stores its (xpm) 
icons in ~/icons (a directory created by my installer). My library 
accesses these by e.g.
(tool-bar-add-item "~/icons/lxa-pin"
Linux Emacs 27.1 is the only version in which this does not work. To 
test whether it does not like that the directory is relatively insecure, 
I tried moving my icons to /usr/local but they were still ignored. In 
all other versions, adding the xpm extension to the reference causes it 
to fail but I tested this anyway and it accomplished nothing.