faces: Fix 24-hour and end-at-midnight timespans

In cases where a working day either:
- Spans 24 hours
or
- ends at midnight
  
the time span calculation code incorrectly calculates the time of that
day, either stating that the day is 0 minutes long, or worse, is
negative.
  
The following patch addresses this.  Non-working days are specified
using the Python 'None', or 'False' values, rather than specifying the
same start and end time, as the latter will now be interpreted as a
24-hour period.

bzr revid: me@vk4msl.yi.org-20120323052146-vosvz50lg12n7e1i

addons/resource/faces/pcalendar.py

@@ -895,7 +895,16 @@ class Calendar(object):
     def _recalc_working_time(self):
         def slot_sum_time(day):
             slots = self.working_times.get(day, DEFAULT_WORKING_DAYS[day])
-            return sum(map(lambda slot: slot[1] - slot[0], slots))
+            def time_diff(times):
+                (start, end) = times
+                if end == start:
+                    return 24*60    # 24 hours
+
+                diff = end - start
+                if end < start:
+                    diff += (24*60)
+                return diff
+            return sum(map(time_diff, slots))
 
         self.day_times = map(slot_sum_time, range(0, 7))
         self.week_time = sum(self.day_times)