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 modification (e.g. commenting a policy, pasting
for first time, selecting whole line and deleting it, ....);
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

@@ -631,7 +631,7 @@ def _text_changed(self, *_args):
         else:
             self.main_window.set_title(self.window_title)
 
-        if not errors and self.source_buffer.get_modified():
+        if not errors:
             self.action_items["save"].set_enabled(True)
             self.action_items["save_exit"].set_enabled(True)
         else: