appdata: Improve appdata for AppStream 1.0 (#222)

* appdata: Improve appdata for AppStream 1.0

- Add the `<developer><name>` tag
- Mark the `<developer_name>` tag as deprecated
- Improve appstreamcli parameters
- Activate meson tests on Flatpak manifest

* Change developer ID to kramo.hu

---------

Co-authored-by: kramo <93832451+kra-mo@users.noreply.github.com>

data/meson.build

@@ -54,7 +54,11 @@ appstream_file = i18n.merge_file(
 
 appstreamcli = find_program('appstreamcli', required: false)
 if appstreamcli.found()
-  test('Validate appstream file', appstreamcli, args: ['validate', appstream_file])
+  test('Validate appstream file',
+    appstreamcli,
+    args: ['validate', '--no-net', '--explain', appstream_file],
+    workdir: meson.current_build_dir()
+  )
 endif
 
 install_data(