added specific flatpak metadata and updated desktop file

ci/dust3d.desktop

@@ -1,7 +1,10 @@
 [Desktop Entry]
+Version=1.0
 Type=Application
+
 Name=Dust3D
+Comment=A quick 3D modeling tool
+Categories=Graphics;Engineering
 Icon=dust3d
 Exec=dust3d
-Categories=Graphics;
+Terminal=false
-Comment=A quick 3D modeling tool

ci/org.dust3d.dust3d.appdata.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>org.dust3d.dust3d</id>
+  
+  <name>Dust3D</name>
+  <summary>A quick 3D modeling tool</summary>
+  
+  <metadata_license>FSFAP</metadata_license>
+  <project_license>MIT</project_license>
+  
+  <description>
+    <p>
+      Dust3D is a brand new 3D modeling software. It helps you create a 3D watertight base model in seconds. Use it to speed up your character modeling in game making, 3D printing, and so on.
+    </p>
+  </description>
+  
+  <launchable type="desktop-id">org.dust3d.dust3d.desktop</launchable>
+  <screenshots>
+    <screenshot type="default">
+      <image>https://raw.githubusercontent.com/huxingyi/dust3d/master/ci/screenshot.png</image>
+    </screenshot>
+  </screenshots>
+</component>

org.dust3d.dust3d.json

@@ -59,7 +59,10 @@
                 "cd thirdparty/instant-meshes/build && make -j $FLATPAK_BUILDER_N_JOBS",
                 "qmake -config release",
                 "make CXX=\"g++ -fext-numeric-literals\"",
-                "install -D dust3d /app/bin/dust3d"
+                "install -Dm755 dust3d -t /app/bin",
+                "install -Dm644 ./ci/org.dust3d.dust3d.appdata.xml -t /app/share/metainfo",
+                "install -Dm644 ./ci/dust3d.desktop /app/share/applications/${FLATPAK_ID}.desktop",
+                "install -Dm644 ./ci/dust3d.png /app/share/icons/hicolor/scalable/apps/${FLATPAK_ID}.png"
             ],
             "sources": [
                 {