We split GUI into a subpackage solaar-gui. /usr/bin/solaar is used both for CLI
commands and starting GUI. This patch adds a helpful error message which is
printed when the user tries to start GUI and solaar-gui is not installed.

--- a/lib/solaar/gtk.py
+++ b/lib/solaar/gtk.py
@@ -149,6 +149,11 @@
         # if any argument, run comandline and exit
         return _cli.run(args.action, args.hidraw_path)
 
+    import importlib.util
+    if not importlib.util.find_spec('solaar.ui'):
+        import sys
+        sys.exit('%s: GUI is not installed; you can install it using: apk add solaar-gui' % NAME)
+
     gi = _require('gi', 'python3-gi (in Ubuntu) or python3-gobject (in Fedora)')
     _require('gi.repository.Gtk', 'gir1.2-gtk-3.0', gi, 'Gtk', '3.0')