ui:Set default value to timedelta arg in add_alarm()

Fixes #1338: Adding an alarm in ikhal causes a crash

> When adding an alarm in ikhal to an existing or new event the program
crashes.

> `TypeError: AlarmsEditor.add_alarm() missing 1 required positional argument: 'timedelta'`

khal/ui/widgets.py

@@ -562,7 +562,7 @@ def clear(self) -> None:
         """clear the alarm list"""
         self.pile.contents.clear()
 
-    def add_alarm(self, button, timedelta: Optional[dt.timedelta]):
+    def add_alarm(self, button, timedelta: Optional[dt.timedelta] = None):
         if timedelta is None:
             timedelta = dt.timedelta(0)
         self.pile.contents.insert(