Include Linux metadata based on App Stream standard

dist/linux/share/applications/org.dhewm3.Dhewm3.d3xp.desktop

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+[Desktop Entry]
+Comment=dhewm 3 is a Doom 3 GPL source port
+Categories=Game;Shooter;
+Exec=dhewm3 +set fs_game d3xp "$@"
+Icon=org.dhewm3.Dhewm3
+Name=Dhewm3 XP
+StartupNotify=true
+Terminal=false
+Type=Application
+Keywords=doom;first;person;shooter;dark;singleplayer;multiplayer;

dist/linux/share/applications/org.dhewm3.Dhewm3.desktop

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+[Desktop Entry]
+Comment=dhewm 3 is a Doom 3 GPL source port
+Categories=Game;Shooter;
+Exec=dhewm3 "$@"
+Icon=org.dhewm3.Dhewm3
+Name=Dhewm3
+StartupNotify=true
+Terminal=false
+Type=Application
+Keywords=doom;first;person;shooter;dark;singleplayer;multiplayer;

dist/linux/share/metainfo/org.dhewm3.Dhewm3.metainfo.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Copyright 2021 Kevin Degeling -->
+
+<component type="desktop">
+  <id>org.dhewm3.Dhewm3</id>
+  <name>Dhewm 3</name>
+  <summary>dhewm 3 is a Doom 3 GPL source port</summary>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-3.0</project_license>
+
+  <developer_name>Dhewm 3 team</developer_name>
+  <update_contact>eonfge@kevindegeling.nl</update_contact>
+
+  <url type="homepage">https://dhewm3.org</url>
+  <url type="help">https://github.com/dhewm/dhewm3/wiki/FAQ</url>
+  <url type="bugtracker">https://github.com/dhewm/dhewm3</url>
+
+  <launchable type="desktop-id">org.dhewm3.Dhewm3.desktop</launchable>
+
+  <description>
+    <p>dhewm3 is a source port of the original Doom3 (not Doom3 BFG, for that you may want to try RBDoom3BFG). It's known to work on Windows, Linux, macOS, FreeBSD, OpenBSD and AROS, but it should work on (or be easily portable to) any system that supports OpenGL 1.4 with ARB shaders, SDL and OpenAL.
+    </p>
+    <p>Compared to the original version of Doom3, dhewm3 has many bugfixes, supports EAX-like sound effects on all operating systems and hardware (via OpenAL Softs EFX support), has much better support for widescreen resolutions and has 64bit support.
+    </p>
+    <p>It only supports old Mods if they either don't require their own game DLL or have been ported to dhewm3 - see the Mods page for more information.
+    </p>
+    <p>Note that while the Doom3 source code has been released under GPL, you still need to legally own the game and provide dhewm3 the game data to play. See the How to Install section for more information.
+    </p>
+  </description>
+
+  <screenshots>
+    <screenshot type="default">
+      <image>https://github.com/dhewm/dhewm.github.io/raw/master/dhewm3-linux.jpg</image>
+      <caption>Doom 3 running on Linux</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://github.com/dhewm/dhewm.github.io/raw/master/dhewm3-1.jpg</image>
+      <caption>Gameplay of Doom 3</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://github.com/dhewm/dhewm.github.io/raw/master/dhewm3-d3xp1.jpg</image>
+      <caption>Gameplay of Doom 3 Expansion pack</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://github.com/dhewm/dhewm.github.io/raw/master/dhewm3-150.jpg</image>
+      <caption>Classic Doom 3 mod support</caption>
+    </screenshot>
+  </screenshots>
+
+  <releases>
+    <release version="1.5.1" date="2021-03-14"/>
+  </releases>
+
+  <!--
+    ESRB gave Doom 3 VR Edition for the PlayStation 4 also a 'Strong language' tag, but no other version has that.
+    Guess that the demons are quite susceptible to motion sickness.
+  -->
+  <content_rating type="oars-1.1">
+    <content_attribute id="violence-realistic">intense</content_attribute>
+    <content_attribute id="violence-bloodshed">intense</content_attribute>
+    <content_attribute id="violence-desecration">intense</content_attribute>
+    <content_attribute id="social-chat">intense</content_attribute>
+    <content_attribute id="social-audio">intense</content_attribute>
+  </content_rating>
+</component>
+