Add key bindings to go to the previous/next month/year

In addition to generic key bindings for moving one day (week)
forward/backward, define similar bindings for moving a month or a year.
Of course, count prefixes are allowed here as well.

Also add status bar hints and help texts.

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

src/calendar.c

@@ -626,6 +626,18 @@ void calendar_move(enum move move, int count)
   case WEEK_NEXT:
     ret = date_change(&t, 0, count * WEEKINDAYS);
     break;
+  case MONTH_PREV:
+    ret = date_change(&t, -count, 0);
+    break;
+  case MONTH_NEXT:
+    ret = date_change(&t, count, 0);
+    break;
+  case YEAR_PREV:
+    ret = date_change(&t, -count * YEARINMONTHS, 0);
+    break;
+  case YEAR_NEXT:
+    ret = date_change(&t, count * YEARINMONTHS, 0);
+    break;
   case WEEK_START:
     /* Normalize struct tm to get week day number. */
     mktime(&t);