Fix RuntimeError: could not create signal for _closing

On Ubuntu 20.04 with GTK 3.24.13, importing sugar3.activity.activity
fails with RuntimeError: could not create signal for _closing, preceeded
by an assertion is_valid_signal_name failed.

Cause is upstream 89f955d ("gsignal: Canonicalise signal names at
installation time ") which requires application signal names to not
start with a hyphen or underscore.

Remove the prefix from our signal name.  The signal remains reserved and
private to the Activity class.

src/sugar3/activity/activity.py

@@ -328,7 +328,7 @@ class Activity(Window, Gtk.Container):
         'joined': (GObject.SignalFlags.RUN_FIRST, None, ([])),
         # For internal use only, use can_close() if you want to perform extra
         # checks before actually closing
-        '_closing': (GObject.SignalFlags.RUN_FIRST, None, ([])),
+        'closing': (GObject.SignalFlags.RUN_FIRST, None, ([])),
     }
 
     def __init__(self, handle, create_jobject=True):
@@ -1301,7 +1301,7 @@ class Activity(Window, Gtk.Container):
 
     def _do_close(self, skip_save):
         self.busy()
-        self.emit('_closing')
+        self.emit('closing')
         if not self._closing:
             if not self._prepare_close(skip_save):
                 return

src/sugar3/activity/widgets.py

@@ -189,7 +189,7 @@ class TitleEntry(Gtk.ToolItem):
         self.add(self.entry)
 
         activity.metadata.connect('updated', self.__jobject_updated_cb)
-        activity.connect('_closing', self.__closing_cb)
+        activity.connect('closing', self.__closing_cb)
 
     def __activate_cb(self, entry, activity):
         self.save_title(activity)