Add Desktop files (#1261)

* Add appstream and desktop files

* More metadata in appstream file

qt/i2pd_qt/data/i2pd.appdata.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Copyright 2018 Viktor Villainov <supervillain@riseup.net> -->
+<component type="desktop">
+  <id>i2pd.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>BSD 3-Clause</project_license>
+  <name>i2pd</name>
+  <summary>Invisible Internet</summary>
+  <description>
+        <p>i2pd (I2P Daemon) is a full-featured C++ implementation of I2P client.</p>
+        <p>I2P (Invisible Internet Protocol) is a universal anonymous network layer. 
+        All communications over I2P are anonymous and end-to-end encrypted, participants
+        don't reveal their real IP addresses. </p>
+        <p>I2P client is a software used for building and using anonymous I2P 
+        networks. Such networks are commonly used for anonymous peer-to-peer 
+        applications (filesharing, cryptocurrencies) and anonymous client-server 
+        applications (websites, instant messengers, chat-servers).</p>
+        <p>I2P allows people from all around the world to communicate and share information
+        without restrictions.</p>
+        <h2>Features</h2>
+        <ul>
+            <li>Distributed anonymous networking framework</li>
+            <li>End-to-end encrypted communications</li>
+            <li>Small footprint, simple dependencies, fast performance</li>
+            <li>Rich set of APIs for developers of secure applications</li>
+        </ul>
+  </description>
+  <screenshots>
+    <screenshot type="default">
+      <image height="590" width="923">https://i2pd.website/images/i2pd_qt.png</image>
+    </screenshot>
+  </screenshots>
+  <url type="homepage">https://i2pd.website/</url>
+  <url type="bugtracker">https://github.com/PurpleI2P/i2pd/issues</url>
+  <url type="help">https://i2pd.readthedocs.io/en/latest/</url>
+  <update_contact>supervillain@riseup.net</update_contact>
+  <developer_name>PurpleI2P Team</developer_name>
+
+  <releases>
+      <release version="2.21.1" date="2018-10-22" />
+      <release version="2.21.0" date="2018-10-04" />
+  </releases>
+  <content_rating type="oars-1.1" />
+</component>

qt/i2pd_qt/data/i2pd.desktop

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+[Desktop Entry]
+Categories=Network;P2P;Qt;
+Exec=i2pd_qt
+GenericName=Invisible Internet
+Comment=A universal anonymous network layer
+Icon=i2pd
+Name=i2pd
+Terminal=false
+Type=Application
+StartupNotify=false
+Keywords=i2p;i2pd;vpn;p2p;