FreeDesktop: Add Release information to AppStream

This commit improves Blender's AppStream metadata with minor punctuation and grammar improvements to the description, and a new `<release>` tag for the current release.

The `<release>` tag benefits Blender in the following ways:
- Flathub users get to see the actual release number instead of "stable"
- Make it easy for all packagers to include changelogs, so Blender's users gets to see what's new in 2.79
- Blender gets featured in GNOME software to reward it for having Release information in its AppStream metadata (KDE Discover [[ https://bugs.kde.org/show_bug.cgi?id=389509 | plans do do the same thing ]])

If the prior objection to adding release data to the AppStream metadata was the ongoing maintenance burden, I volunteer to take it on for subsequent Blender releases.

Author: @ngraham

Differential Revision: https://developer.blender.org/D3065

release/freedesktop/org.blender.Blender.appdata.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <component type="desktop">
     <id>org.blender.Blender</id>
-    <launchable type="desktop">blender.desktop</launchable>
+    <launchable type="desktop-id">blender.desktop</launchable>
     <name>Blender</name>
     <summary>Free and open source 3D creation suite</summary>
     <metadata_license>CC0-1.0</metadata_license>
@@ -9,19 +9,19 @@
     <description>
         <p>
             Blender is the free and open source 3D creation suite. It supports
-            the entirety of the 3D pipeline—modeling, rigging, animation,
+            the entirety of the 3D pipeline — modeling, rigging, animation,
             simulation, rendering, compositing and motion tracking, even video
             editing and game creation.
         </p>
 
         <p>
-            Advanced users employ Blender’s API for Python scripting to
+            Advanced users can employ Blender’s API for Python scripting to
             customize the application and write specialized tools. Blender is
             well suited to individuals and small studios who benefit from its
             unified pipeline and responsive development process.
         </p>
     </description>
-    <url type="homepage">http://www.blender.org</url>
+    <url type="homepage">https://www.blender.org</url>
     <url type="help">https://www.blender.org/support/</url>
     <url type="bugtracker">https://developer.blender.org</url>
     <url type="donation">https://www.blender.org/foundation/donation-payment/</url>
@@ -39,4 +39,31 @@
             <image>https://download.blender.org/demo/screenshots/blender_screenshot_4.jpg</image>
         </screenshot>
     </screenshots>
+    <releases>
+        <release version="2.79" date="2017-09-12">
+            <description>
+                <p>New features:</p>
+                <ul>
+                    <li>Denoiser</li>
+                    <li>Filmic color management</li>
+                    <li>PBR shader</li>
+                    <li>Shadow catcher</li>
+                    <li>Surface Deform modifier</li>
+                    <li>More bundled add-ons</li>
+                    <li>...And many more!</li>
+                </ul>
+                <p>Enhancements:</p>
+                <ul>
+                    <li>Faster OpenCL</li>
+                    <li>Cycles improvements</li>
+                    <li>Modeling tool improvements</li>
+                    <li>Animation improvements</li>
+                    <li>Grease Pencil improvements</li>
+                    <li>Automatic scaling on high DPI displays</li>
+                    <li>Alembic import and export improvements</li>
+                    <li>...And many more!</li>
+                </ul>
+            </description>
+        </release>
+  </releases>
 </component>