year-view: Reduce maximum of month columns

The view had a maximum of 6 month columns, leading to a huge amount of
vertical spaces, so it was requested to reduce that.

The maximum number of columns was reduced to 4, so the view will have a
maximum 4x3 months layout.

https://bugzilla.gnome.org/show_bug.cgi?id=753533

src/gcal-year-view.c

@@ -25,7 +25,7 @@
 #include <math.h>
 #include <string.h>
 
-#define NAVIGATOR_MAX_GRID_SIZE 6
+#define NAVIGATOR_MAX_GRID_SIZE 4
 #define VISUAL_CLUES_SIDE 3.0
 #define WEEK_NUMBER_MARGIN 3.0