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/calcurse.h

@@ -403,6 +403,10 @@ enum key {
   KEY_GENERIC_NEXT_DAY,
   KEY_GENERIC_PREV_WEEK,
   KEY_GENERIC_NEXT_WEEK,
+  KEY_GENERIC_PREV_MONTH,
+  KEY_GENERIC_NEXT_MONTH,
+  KEY_GENERIC_PREV_YEAR,
+  KEY_GENERIC_NEXT_YEAR,
   KEY_GENERIC_SCROLL_DOWN,
   KEY_GENERIC_SCROLL_UP,
   KEY_GENERIC_GOTO_TODAY,
@@ -539,7 +543,11 @@ enum move {
   WEEK_PREV,
   WEEK_NEXT,
   WEEK_START,
-  WEEK_END
+  WEEK_END,
+  MONTH_PREV,
+  MONTH_NEXT,
+  YEAR_PREV,
+  YEAR_NEXT
 };
 
 /* Available color pairs. */