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.

NOTE 1: This change could potentially impact the darwin package as it
removes an explicit isDarwin check related to GIO_EXTRA_MODULES. The
Darwin case is hopefully handled correctly by conditional code in
wrapGAppsHook. Testing required.

NOTE 2: As with other packages wrapped by wrapGAppsHook, the wrapping
introduces an extra level of wrapping when it encounters a soft link
to an executable in bin. In this package, there is a soft link
from gnumeric to gnumeric-1.12.44. After wrapping with wrapGAppsHook,
the execution flow for gnumeric looks like this:

gnumeric execs .gnumeric-wrapped which is a link to
gnumeric-1.12.44 that then execs .gnumeric-1.12.44-wrapped

This may not be optimal, but seems harmless, and could be fixed by
a future update to wrapGAppsHook.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.