Fix first file SEGV

If you delete and readd an activity after showing it on
the diary view you get a SEGV.

This is an edge case related to deleting the last activity
and going back to the blank state before importing another.

Actually there are 3 technical issues;

1. GcCalendar doesn't get notified by MainWindow when
   there are no rides -- so it crashes on refresh

2. RideSummaryWindow doesn't get notified by home window
   if its not visible

3. RideSummaryWindow should check rideItem isn't NULL before
   trying to plot zones (but only for date range summaries)

Fixes #622
Diary Sidebar Calendar ignores first ride

Fix to check the first (0th) ride when clicking left
for previous ride rather than >0th.
Only refresh calendar if visible

Speeds things up and also removes problem of refreshing
when referencing a deleted ride.
Fix delete ride SEGV

Ridesummarywindow was using rideitem to get zones to use when
summarising for a date range which was bad because a) the ride
might not be for the date range selected and b) it might have
been deleted.

This fixes that and means we get a summary that is more accurate
and no crashing when rides are deleted.
Warning on add event

.. to explain they belong to a season.
Merge branch 'master' of github.com:GoldenCheetah/GoldenCheetah