add app metainfo (#1535)

* add appdata

* move appdata into data subdir

* appdata: id change

data/appdata.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop">
+  <id>studio.kx.carla</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-2.0</project_license>
+  <name>Carla</name>
+  <summary>Audio Plugin Host</summary>
+  <description>
+    <p>Carla is a fully-featured audio plugin host, with support for many audio drivers and plugin formats.</p>
+  </description>
+  <screenshots>
+    <screenshot type="default">
+      <image>https://kx.studio/screenshots/carla.png</image>
+      <image>https://kx.studio/screenshots/carla-patchbay.png</image>
+    </screenshot>
+  </screenshots>
+  <content_rating type="oars-1.1" />
+  <launchable type="desktop-id">studio.kx.carla.desktop</launchable>
+  <url type="homepage">https://kx.studio/Applications:Carla</url>
+  <url type="bugtracker">https://github.com/falkTX/Carla/issues</url>
+  <releases>
+    <release version="2.4.1" date="2021-10-15" />
+  </releases>
+</component>