Fix opening links in MUC subject

The default GTK link handler does not work on Windows and Mac

Fixes #9453

gajim/groupchat_control.py

@@ -54,6 +54,7 @@
 from gajim.common import events
 from gajim.common import app
 from gajim.common import helpers
+from gajim.common.helpers import launch_browser_mailer
 from gajim.common.modules import dataforms
 from gajim.common import ged
 from gajim.common import i18n
@@ -3050,6 +3051,7 @@ def __init__(self):
         self.label.set_line_wrap(True)
         self.label.set_line_wrap_mode(Pango.WrapMode.WORD_CHAR)
         self.label.set_max_width_chars(80)
+        self.label.connect('activate-link', self._on_activate_link)
 
         scrolledwindow.add(self.label)
 
@@ -3069,3 +3071,10 @@ def _after_show(self, *args):
         # So we switch after show to False and again to True
         self.label.set_selectable(False)
         self.label.set_selectable(True)
+
+    @staticmethod
+    def _on_activate_link(_label, uri):
+        # We have to use this, because the default GTK handler
+        # is not cross-platform compatible
+        launch_browser_mailer(None, uri)
+        return Gdk.EVENT_STOP