widgets: checkbox: Add a label property

The checkbox uses the _im(mutable) idiom to minimize the RAM overhead
of its immutable properties (position and label). However it can be useful
to retrieve the label to provide a property accessor.

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

wasp/widgets.py

@@ -221,6 +221,10 @@ class Checkbox():
         self._im = (x, y, label)
         self.state = False
 
+    @property
+    def label(self):
+        return self._im[2]
+
     def draw(self):
         """Draw the checkbox and label."""
         draw = wasp.watch.drawable