Implement a cache for the monthly view

Add a very simple cache, which is used to store the days that contain an
event or an appointment. This makes redrawing and browsing the calendar
panel much faster.

The cache has a size of 31 integers (which is equivalent to 124 bytes on
a 32 bit system and 248 bytes on a 64 bit system) and invalidates itself
if the current month has changed. If an item is added/changed/removed,
the cache needs to be invalidated manually by calling
calendar_monthly_view_cache_set_invalid(). Note that this will always
invalidate the whole cache, even if only one item at the last day of the
month was removed. This is a trade-off between simplicity and
efficiency.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.c

@@ -369,6 +369,7 @@ int main(int argc, char **argv)
     case KEY_GENERIC_IMPORT:
       wins_erase_status_bar();
       io_import_data(IO_IMPORT_ICAL, NULL);
+      calendar_monthly_view_cache_set_invalid();
       inday = do_storage(0);
       wins_update(FLAG_ALL);
       break;