Prevent gettext from extracting Icon field in .desktop

Newer versions of gettext recognize that Icon field is not something that should be translated in .po files [1][2]. However, gettext in Focal doesn't recognize that yet. So we have to tell gettext explicitly.

Unfortunately, xgettext and msgfmt have the way to disable all built-in keywords, but not a single one. Thus, we have to specify each keyword ourselves.

[1] https://www.gnu.org/software/gettext/manual/html_node/Icons.html
[2] https://github.com/autotools-mirror/gettext/commit/a5004837372309ce19e40b2ccbe5293dfe268329

Fixes: #100 (closed)
Bug: #107 (closed)