Incorporate wrapGAppsHook so that all gnumeric binaries are wrapped,
following the convention used by many gnome applications.
This addresses two issues:
1. The packaged ssconvert, ssdiff, ssgrep, and ssindex executables
in bin are not currently wrapped so some expected environment
variables including XDG_DATA_DIRS and GIO_EXTRA_MODULES are not
set. The result is many warnings on stderr when running these
commands, e.g.
==================================================================
CRITICAL **:...go_conf_add_monitor: assertion 'node || key' failed
CRITICAL **:...go_conf_get_node: assertion 'parent || key' failed
WARNING **:...unknown GOConfMonitor id.
==================================================================
2. None of the binaries, including gnumeric, currently wrap the
environment variable GDK_PIXBUF_MODULE_FILE. This can cause
segfaults if an incompatible GDK_PIXBUF_MODULE_FILE is already set
in the environment (e.g. by plasma5). This could be encountered
running a nixos pre-19.03 gnumeric binary from a nixos 18.09 KDE
session.
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