window: replace New Event with Today button

As Lapo rightfully explain, providing a button there for creating a new
event is way to prominently and might divert users of the intended click
on a cell behavior to create an event.
New event will be also accessible from the hamburger icon on the left of
the header-bar and Ctrl-N key shortcut.
Also as Lapo suggestion Today button fall better on the header bar than
between the navigation buttons.

src/gcal-nav-bar.c

@@ -26,7 +26,6 @@
 typedef struct
 {
   GtkWidget *back_button;
-  GtkWidget *today_button;
   GtkWidget *forward_button;
 
   GtkWidget *left_label;
@@ -89,7 +88,6 @@ gcal_nav_bar_class_init (GcalNavBarClass *klass)
 
   /* Bind internals widgets */
   gtk_widget_class_bind_template_child_private (widget_class, GcalNavBar, back_button);
-  gtk_widget_class_bind_template_child_private (widget_class, GcalNavBar, today_button);
   gtk_widget_class_bind_template_child_private (widget_class, GcalNavBar, forward_button);
   gtk_widget_class_bind_template_child_private (widget_class, GcalNavBar, left_label);
   gtk_widget_class_bind_template_child_private (widget_class, GcalNavBar, right_label);
@@ -176,15 +174,6 @@ gcal_nav_bar_get_prev_button (GcalNavBar *nav_bar)
   return priv->back_button;
 }
 
-GtkWidget*
-gcal_nav_bar_get_today_button (GcalNavBar *nav_bar)
-{
-  GcalNavBarPrivate *priv;
-
-  priv = gcal_nav_bar_get_instance_private (nav_bar);
-  return priv->today_button;
-}
-
 GtkWidget*
 gcal_nav_bar_get_next_button (GcalNavBar *nav_bar)
 {