Fix error saving to a different position type format

This is expected fallout from porting from Gtk.UIManager etc. to
Gio.Action etc., which changed radio and toggle action handling.

Closes #28

TODO.md

@@ -0,0 +1,7 @@
+Gaupol 0.92/1.0
+===============
+
+* [x] Fix error saving document from a time-based format to a
+      frame-based or vice versa ([#28][])
+
+[#28]: https://github.com/otsaloma/gaupol/issues/28

gaupol/agents/save.py

@@ -184,8 +184,7 @@ def _select_file(self, title, page, file=None):
             # Set framerate to the selected one.
             self.set_current_page(page)
             action = self.get_action("set-framerate")
-            action.set_state(str(framerate))
-            action.activate()
+            action.activate(str(framerate))
         gaupol.util.iterate_main()
         return aeidon.files.new(format, path, encoding, newline)