Don't extract Icon values from .desktop files into POT files.

Don't extract Icon values from .desktop files into POT files.

Reported by Piotr Drąg <piotrdrag@gmail.com>
at <https://savannah.gnu.org/support/?108887>
and by Will Thompson <wjt@endlessm.com>
at <https://savannah.gnu.org/bugs/?56543>.

There was a discussion starting at
<https://lists.freedesktop.org/archives/xdg/2019-June/014158.html>.
The most interesting posts in this thread were
<https://lists.freedesktop.org/archives/xdg/2019-June/014164.html>
<https://lists.freedesktop.org/archives/xdg/2019-June/014167.html>.

* gettext-tools/doc/gettext.texi (Icons): New subsubsection.
* gettext-tools/src/read-desktop.c (desktop_add_default_keywords): Don't add
"Icon".
* gettext-tools/src/x-desktop.c: Update comments.
* gettext-tools/doc/xgettext.texi (Desktop): Remove 'Icon' from the list of
extracted keys.
* gettext-tools/tests/xgettext-desktop-2: New file.
* gettext-tools/tests/msgfmt-desktop-3: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
* NEWS: Mention the change.