Appstream updates part 1

* Add brand colors to appstream
* Use non-deprecated developer tag and update screenshot urls

Author: user1-github

src/install/nix/blue-nebula.appdata.xml.am

@@ -0,0 +1,54 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>@APPNAME@.desktop</id>
+  <launchable type="desktop-id">@APPNAME@.desktop</launchable>
+  <metadata_license>CC-BY-SA-3.0+</metadata_license>
+  <project_license>Zlib and CC-BY-SA-3.0+</project_license>
+  <name>Blue Nebula</name>
+  <developer id="org.blue-nebula">
+    <name>Blue Nebula Team</name>
+  </developer>
+  <branding>
+    <color type="primary" scheme_preference="light">#1970c2</color>
+    <color type="primary" scheme_preference="dark">#0a2a48</color>
+  </branding>
+  <summary>Free agile arena shooter</summary>
+  <description>
+    <p>
+      Blue Nebula is a fun-filled new take on the first person arena shooter,
+      built as a total conversion of Cube Engine 2, which lends itself toward
+      a balanced gameplay, with a general theme of agility in a variety of
+      environments. It features a unique parkour movement system.
+    </p>
+    <ul>
+      <li>Cross-platform multiplayer</li>
+      <li>Features parkour, impulse boosts, dashing, and other tricks</li>
+      <li>Favourite gamemodes with an array of mutators and variables</li>
+      <li>Built-in editor lets you create your own maps cooperatively online</li>
+    </ul>
+  </description>
+  <url type="homepage">https://blue-nebula.org</url>
+  <url type="bugtracker">https://github.com/blue-nebula/base/issues</url>
+  <screenshots>
+    <screenshot type="default">
+      <image>https://raw.githubusercontent.com/blue-nebula/misc/master/screenshots/1.jpg</image>
+      <caption>Posing with fire on the map Venus</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://raw.githubusercontent.com/blue-nebula/misc/master/screenshots/2.jpg</image>
+      <caption>Mine fireworks on the map Vault</caption>
+    </screenshot>
+    <screenshot>
+      <image>https://raw.githubusercontent.com/blue-nebula/misc/master/screenshots/3.jpg</image>
+      <caption>Overlooking friendly fire on the map Cutec</caption>
+    </screenshot>
+  </screenshots>
+  <content_rating type="oars-1.1">
+    <content_attribute id="violence-fantasy">moderate</content_attribute>
+    <content_attribute id="violence-bloodshed">mild</content_attribute>
+    <content_attribute id="social-chat">intense</content_attribute>
+  </content_rating>
+  <releases>
+    <release version="1.6.0" date="2017-12-21"/>
+  </releases>
+</component>