Add AppStream metadata

[AsciiWolf]

This PR adds AppStream metadata that are used by modern package management systems nowadays and required by GUI software centers such as GNOME Software or KDE Discover.

Closes #129.
Hopefully will also finally close BTS#1091522.

The changes from this PR are tested and confirmed to work fine.

Compiled locally on my Debian testing machine using:

sudo apt-get build-dep synaptic
./autogen.sh
./configure --prefix=/usr
make -j32
sudo make install

Confirmed that the AppStream metainfo file is installed correctly:

$ cd /usr/share/metainfo/
$ ls -l | grep io.github.mvo5.synaptic
-rw-r--r-- 1 root root   1567 Jan  9 01:36 io.github.mvo5.synaptic.metainfo.xml
$ file io.github.mvo5.synaptic.metainfo.xml
io.github.mvo5.synaptic.metainfo.xml: XML 1.0 document, ASCII text
$ cat io.github.mvo5.synaptic.metainfo.xml
<?xml version="1.0" encoding="UTF-8"?>
<component type="desktop-application">
  <id>io.github.mvo5.synaptic</id>
  <launchable type="desktop-id">synaptic.desktop</launchable>
  <metadata_license>CC0-1.0</metadata_license>
  <project_license>GPL-2.0</project_license>
  <name>Synaptic</name>
  <developer_name>Michael Vogt</developer_name>
  <developer id="io.github.mvo5">
    <name>Michael Vogt</name>
  </developer>
  <summary>Graphical package manager</summary>
  <description>
...

P. S. I did not make the metainfo file translatable because I honestly have no idea how to handle xml files correctly using intltool. This can be added later.

[AsciiWolf]

How it looks like in GNOME Software:

(Ignore the missing screenshot. It will be available after this PR gets merged.)

[AsciiWolf]

@mvo5 Let me know if there is any problem or if you have any question. I have quite a bit of experience with AppStream metadata and related things. :-)