snap: Take the summary and description from the appdata.xml file

Just avoids a bit of duplication.

snap/snapcraft.yaml

@@ -1,11 +1,5 @@
 name: tiled
 adopt-info: tiled
-summary: Your free, easy to use and flexible tile map editor.
-description: |
-  Tiled is a free software level editor. It supports editing tile maps in
-  various projections (orthogonal, isometric, hexagonal) and also supports
-  building levels with freely positioned, rotated or scaled images or
-  annotating them with objects of various shapes.
 license: GPL-2.0
 base: core18
 
@@ -15,7 +9,7 @@ confinement: strict
 apps:
   tiled:
     command: desktop-launch ${SNAP}/usr/bin/tiled
-    desktop: usr/share/applications/org.mapeditor.Tiled.desktop
+    common-id: org.mapeditor.Tiled.desktop
     plugs: [home, opengl, unity7, network]
   tmxviewer:
     command: desktop-launch ${SNAP}/usr/bin/tmxviewer
@@ -35,6 +29,8 @@ parts:
       - PREFIX=/usr
       - SYSTEM_ZSTD=yes
       - CONFIG+=release
+    parse-info:
+      - org.mapeditor.Tiled.appdata.xml
     override-pull: |
       snapcraftctl pull
       snapcraftctl set-version "$(git describe | cut -c 2-)"