Bug#849249: fldigi: Icon improvements

[Jeremy Bicha <jbicha@ubuntu.com>]

Package: fldigi
Version: 3.23.14-1
Tags: patch

.xpm app icons are not necessary any more. If a package contains .xpm
icons, it triggers an error in appstream. This means that fldigi does
not show up in the GNOME Software app which is installed by default in
the GNOME desktop in Debian stretch.

https://appstream.debian.org/sid/main/issues/fldigi.html
https://wiki.debian.org/AppStream/Guidelines#Icon_best_practices

The .png app icons are installed in the wrong directory. They should
be installed in a two-number directory like 64x64/ instead of a
one-number directory like 64/

There's some extra line in debian/rules. The dh_installdocs override
isn't needed since there's only one binary package.
--disable-silent-rules is the default as of debhelper 9.20150501. And
dpkg-buildflags should be passed automatically without you having to
do anything.

I'm attaching patches for these.

Thanks,
Jeremy