Use the correct icon extension

See:
- https://docs.flathub.org/docs/for-app-authors/linter/#non-png-icon-in-hicolor-size-folder
- https://docs.flathub.org/docs/for-app-authors/linter/#non-svg-icon-in-scalable-folder

Author: JoseExposito

data/icons/128/app.png

@@ -93,13 +93,14 @@ icon_sizes = ['16', '24', '32', '48', '64', '128']
 
 foreach i : icon_sizes
     install_data(
-        join_paths('icons', i, 'app.svg'),
-        rename: meson.project_name() + '.svg',
+        join_paths('icons', i, 'app.png'),
+        rename: meson.project_name() + '.png',
         install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', i + 'x' + i, 'apps')
     )
-    install_data(
-        join_paths('icons', i, 'app.svg'),
-        rename: meson.project_name() + '.svg',
-        install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', i + 'x' + i + '@2', 'apps')
-    )
 endforeach
+
+install_data(
+  join_paths('icons', 'scalable', 'app.svg'),
+  rename: meson.project_name() + '.svg',
+  install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', 'scalable', 'apps')
+)