Added empty implementation of set_on_exit to dummy, gtk and cocoa

Signed-off-by: obulat <[email protected]>

src/cocoa/toga_cocoa/app.py

@@ -167,3 +167,6 @@ def main_loop(self):
 
     def exit(self):
         self.native.terminate(None)
+
+    def set_on_exit(self):
+        self.interface.factory.not_implemented('App.set_on_exit()')

src/core/toga/app.py

@@ -75,6 +75,7 @@ def __init__(self, name, app_id,
         self._on_exit = None
 
         self._impl = self.factory.App(interface=self)
+        self.on_exit = on_exit
 
     @property
     def app_id(self):

src/dummy/toga_dummy/app.py

@@ -29,3 +29,7 @@ def main_loop(self):
     @not_required_on('mobile', 'web')
     def exit(self):
         self._action('exit')
+
+    @not_required_on('mobile', 'web')
+    def set_on_exit(self, value):
+        self._set_value('on_exit', value)

src/gtk/toga_gtk/app.py

@@ -153,3 +153,6 @@ def main_loop(self):
 
     def exit(self):
         self.native.quit()
+
+    def set_on_exit(self):
+        self.interface.factory.not_implemented('App.set_on_exit()')