year-view: Added visual clue for days with events

Year view didn't have hints for days with events and this feature was
requested to be added to gnome-calendar.

This is an old request but in the past gnome-design team though it would
be better to keep the layout simple without hints for days with events.
Now with the changes in gnome-shell calendar and in order to keep them
in sync, this feature was added to gnome-calendar.

In order to implement this code we had to add a few css styles and a
code to add a small circle below everyday with events. Although
gnome-shell calendar uses an svg image with a 3x3 dot, we chose to use a
pango function to draw the dot.

https://bugzilla.gnome.org/show_bug.cgi?id=743317

data/theme/gtk-styles.css

@@ -120,6 +120,19 @@ calendar-view.second-view-header {
     font-weight: bold;
 }
 
+.year-navigator.with-events {
+    background-color: @theme_fg_color;
+    border-radius: 3px;
+}
+
+.year-navigator.with-events-selected {
+    background-color: @theme_selected_bg_color;
+}
+
+.year-navigator.with-events-sunday {
+    background-color: alpha(@theme_fg_color, 0.5);
+}
+
 .sidebar-header {
     font-weight: bold;
     font-size: 11px;