apps: Replace del self.x with self.x = None in all apps

Testing has demonstrated that del self.x does not make the memory used to
store x available for garbage collection.

There is clearly an additional reference from another place. In fact
*after* del self.x then the memory can be made available for GC by
assignment (e.g. self.x = None). However I haven't found how to release
this reference and there is nothing in self.__dict__ that can help.

For now we'll use a twp-step process where we set the variable to None
before deleting it.

This has a big impact on memory usage. For Software it is almost 1k
(a.k.a. about 10% impact on free RAM).

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

wasp/apps/alarm.py

@@ -124,10 +124,15 @@ class AlarmApp:
 
         self.page = _HOME_PAGE
 
+        self.del_alarm_btn = None
         del self.del_alarm_btn
+        self.hours_wid = None
         del self.hours_wid
+        self.min_wid = None
         del self.min_wid
+        self.alarm_checks = None
         del self.alarm_checks
+        self.day_btns = None
         del self.day_btns
 
         self._set_pending_alarms()

wasp/apps/faces.py

@@ -37,8 +37,11 @@ class FacesApp():
         wasp.system.request_event(wasp.EventMask.SWIPE_UPDOWN)
 
     def background(self):
+        self.choices = None
         del self.choices
+        self.choice = None
         del self.choice
+        self.si = None
         del self.si
 
         # When the watch face redraws then the change to the scrolling indicator

wasp/apps/software.py

@@ -67,8 +67,11 @@ class SoftwareApp():
                                   wasp.EventMask.SWIPE_UPDOWN)
 
     def background(self):
+        self.si = None
         del self.si
+        self.page = None
         del self.page
+        self.db = None
         del self.db
 
     def get_page(self):