Add FreeDesktop metadata (#87)

* Add a start menu shortcut

* Add app store metadata including age rating

misc/freedesktop/urbanterror.appdata.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<component type="desktop">
+  <id>urbanterror.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-2.0+ AND LicenseRef-proprietary=https://www.urbanterror.info/support/202-quake-3-sdk/</project_license>
+  <developer_name>Frozensand Games Limited</developer_name>
+  <name>Urban Terror</name>
+  <summary>Tactical first person shooter with action movie elements</summary>
+  <description>
+    <p>
+     Urban Terror could best be described as a Hollywood tactical shooter; it is
+     realism based to a certain extent (environments/weapons/player models), but
+     also goes by the motto "fun over realism" (fast gameplay and lots of
+     action). This combination of reality and action results in a very unique,
+     enjoyable and addictive game.
+    </p>
+  </description>
+  <url type="homepage">https://www.urbanterror.info/</url>
+  <screenshots>
+    <screenshot>https://www.urbanterror.info/files/static/images/pics/presentation/shot0001.jpg</screenshot>
+    <screenshot>https://www.urbanterror.info/files/static/images/pics/presentation/shot0007.jpg</screenshot>
+    <screenshot type="default">https://www.urbanterror.info/files/static/images/pics/presentation/shot0016.jpg</screenshot>
+    <screenshot>https://www.urbanterror.info/files/static/images/pics/presentation/shot0017.jpg</screenshot>
+    <screenshot>https://www.urbanterror.info/files/static/images/pics/presentation/shot0018.jpg</screenshot>
+  </screenshots>
+  <content_rating type="oars-1.1">
+    <content_attribute id="violence-realistic">intense</content_attribute>
+    <content_attribute id="violence-bloodshed">moderate</content_attribute>
+    <content_attribute id="social-chat">intense</content_attribute>
+    <content_attribute id="social-info">mild</content_attribute>
+  </content_rating>
+  <releases>
+    <release date="2018-06-22" version="4.3.4" />
+    <release date="2018-03-01" version="4.3.3" />
+    <release date="2017-02-05" version="4.3.2" />
+    <release date="2016-10-06" version="4.3.1" />
+    <release date="2016-10-01" version="4.3.0" />
+  </releases>
+</component>

misc/freedesktop/urbanterror.desktop

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+[Desktop Entry]
+Categories=Game;ActionGame;
+Name=Urban Terror
+GenericName=Tactical FPS
+Type=Application
+Exec=urbanterror
+Icon=urbanterror