apps: calc: Remember the results between invocations

Currently if you spend more then 15 seconds looking up figures or
transcribing the answer then the system will switch back to the
clock and the answer will be lost.

Fix this by remembering the output between invocations.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

wasp/apps/calc.py

@@ -53,7 +53,7 @@ class CalculatorApp():
 
     def foreground(self):
         self._draw()
-        self.output = ""
+        self._update()
         wasp.system.request_event(wasp.EventMask.TOUCH)
 
     def touch(self, event):