Allow user to save valid modified policy on 1st char

Checking for `get_modified()` within signal handler for `changed` signal
does not work for the 1st character (e.g. commenting a policy line)
as the output of `get_modified()` only changes after the signal handler.

fixes: QubesOS/qubes-issues#9911

Author: alimirjamali

qubes_config/policy_editor/policy_editor.py

@@ -385,6 +385,7 @@ def setup_source(self):
         )
         self.source_view.set_monospace(True)
         self.source_buffer.connect("changed", self._text_changed)
+        self.source_buffer.connect("modified-changed", self._text_changed)
         self.source_buffer.get_undo_manager().connect(
             "can-redo-changed", self._redo_changed
         )