#7 Improve Debian package builds

Freedesktop.org's Cross-Desktop Group publishes the 'Icon Theme
Specification'[1], which states that the 48px² icon is strongly
preferred from third-party installs, and though larger icons are
supported, those larger than 256px² should be scaled for HiDPI
environments (in the form of 128x128@2, etc.).

This commit removes the 512px² icon from the build/iconset directory and
adds icons for the XDG standard icon sizes of 16px², 22px², 24px²,
32px², 48px² and 64px². All files have been optimized using pngcrush,
resulting in a filesize reduction of >60% for all files. Images were
also trimmed using ImageMagick 7 to remove null space borders with the
constraint that the output remain with a pageview aspect ratio of 1:1.

Refs:
[1]: https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html

Signed-off-by: Peter J. Mello <admin@petermello.net>

Previously the Debian package builds were lacking several mandatory
fields in their generated DEBIAN/control files, among them 'section', a
valid SPDX License Expression in the License field, valid 'Priority'
value, sub-optimal runtime dependency packages, and a valid Desktop
Entry file which placed the launch entry in the correct category instead
of 'Lost and Found'. The result was a corruption of the apt-xapian-index
on Debian-based Linux distributions and some minor usability issues for
end users. This commit corrects those oversights and builds a Debian
package that only slightly violates Debian Standards Policy v4.5.1.0,
the most recent one published.

Signed-off-by: Peter J. Mello <admin@petermello.net>