year-view: add hover effect to the navigator

The current implementation of the Year navigator gives
absolutely no clue that the days are actually selectable,
which may confuse some users.

A simple way to inform the user that the Year navigator
is actually interactive is the hovering effect.

Fix the lack of feedback by adding the hover effect to the
year navigator.

https://bugzilla.gnome.org/show_bug.cgi?id=762462

data/theme/gtk-styles.css

@@ -122,6 +122,11 @@ calendar-view.second-view-header {
     font-weight: bold;
 }
 
+.year-navigator:hover {
+    background-color: alpha(@theme_fg_color, 0.1);
+    border-radius: 2px;
+}
+
 .year-navigator.with-events {
     background-color: @theme_fg_color;
     border-radius: 3px;