data: Adapt appdata and checks to new flathub standards

* appdata: Remove Purism::form_factor=workstation

This is a key=value dict, with two entries one gets an error:

    E: com.github.tchx84.Flatseal:39: custom-key-duplicated Purism::form_factor
       A key can only be used once.

Apps generally remove the workstation key so the app is still marked as
mobile capable.

* appdata: Add developer tag

developer_name was deprecated in AppStream 1.0, this is the replacement.

* appdata: Add launchable

Fixes the following error:

    com.github.tchx84.Flatseal:~: desktop-app-launchable-missing
    This `desktop-application` component is missing a `desktop-id` launchable tag. This means that
    this application can not be launched and has no association with its desktop-entry file. It also
    means no icon data or category information from the desktop-entry file will be available, which
    will result in this application being ignored entirely.

* meson: Switch tests to appstreamcli

data/com.github.tchx84.Flatseal.appdata.xml.in

@@ -3,6 +3,7 @@
   <!-- TRANSLATORS: Don't translate this text -->
   <name>Flatseal</name>
   <id>com.github.tchx84.Flatseal</id>
+  <launchable type="desktop-id">com.github.tchx84.Flatseal.desktop</launchable>
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-3.0-or-later</project_license>
   <content_rating type="oars-1.1"/>
@@ -32,10 +33,12 @@
   <update_contact>martin.abente.lahaye@gmail.com</update_contact>
   <!-- TRANSLATORS: Don't translate this text -->
   <developer_name>Martin Abente Lahaye</developer_name>
+  <developer id="dev.tchx84">
+    <name translatable="no">Martin Abente Lahaye</name>
+  </developer>
   <url type="homepage">https://github.com/tchx84/flatseal</url>
   <url type="bugtracker">https://github.com/tchx84/flatseal/issues</url>
   <custom>
-    <value key="Purism::form_factor">workstation</value>
     <value key="Purism::form_factor">mobile</value>
   </custom>
 </component>

data/meson.build

@@ -22,10 +22,10 @@ appstream_file = i18n.merge_file(
   install_dir: join_paths(get_option('datadir'), 'appdata')
 )
 
-appstream_util = find_program('appstream-util', required: false)
-if appstream_util.found()
-  test('Validate appstream file', appstream_util,
-    args: ['validate', appstream_file]
+appstreamcli = find_program('appstreamcli', required: false)
+if appstreamcli.found()
+  test('Validate appstream file', appstreamcli,
+    args: ['validate', '--no-net', '--explain', appstream_file]
   )
 endif