ui: Added GcalNavBar widget.

GcalNavBar widget is composited using templates.
Great work of Tristan Van Berkom <tristanvb@openismus.com> on
using this.

data/theme/gtk-styles.css

@@ -4,7 +4,12 @@
   background-image: none;
 }
 
-.calendar-view header {
+.nav-button {
+  background-color: transparent;
+  background-image: none;
+}
+
+.nav-header {
   font: bold 12;
   padding: 12px;
 }
@@ -51,20 +56,6 @@
   border-radius: 0;
 }
 
-GtkClutterOffscreen.overlay {
-  background-color: transparent;
-  background-image: none;
-}
-
-GtkClutterOffscreen.overlay .frame {
-  padding: 12px;
-  margin: 12px;
-}
-
-GtkClutterOffscreen.overlay .frame * {
-  color: @theme_fg_color;
-}
-
 GtkOverlay > GtkButton {
    padding-top: 0px;
    padding-right: 0px;