Make appearance.calendarview more explicit in config file

Instead of using 0 or 1 as a value for `appearance.calendarview`,
introduce the more explicit "monthly" and "weekly".

Also update `scripts/calcurse-upgrade.sh` to reflect the change.

Signed-off-by: Baptiste Jonglez <baptiste--git@jonglez.org>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calendar.c

@@ -57,12 +57,6 @@
 
 #define ISLEAP(y) ((((y) % 4) == 0 && ((y) % 100) != 0) || ((y) % 400) == 0)
 
-enum {
-  CAL_MONTH_VIEW,
-  CAL_WEEK_VIEW,
-  CAL_VIEWS
-};
-
 enum pom {
   NO_POM,
   FIRST_QUARTER,