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.

data/ui/nav_bar.ui

@@ -37,28 +37,6 @@
             <property name="position">0</property>
           </packing>
         </child>
-        <child>
-          <object class="GtkButton" id="today_button">
-            <property name="visible">True</property>
-            <property name="can_focus">True</property>
-            <property name="focus_on_click">False</property>
-            <style>
-              <class name="nav-button"/>
-            </style>
-            <child>
-              <object class="GtkLabel" id="today_label">
-                <property name="visible">True</property>
-                <property name="can_focus">False</property>
-                <property name="label" translatable="yes">Today</property>
-              </object>
-            </child>
-          </object>
-          <packing>
-            <property name="expand">False</property>
-            <property name="fill">True</property>
-            <property name="position">1</property>
-          </packing>
-        </child>
         <child>
           <object class="GtkButton" id="forward_button">
             <property name="visible">True</property>
@@ -78,7 +56,7 @@
           <packing>
             <property name="expand">False</property>
             <property name="fill">True</property>
-            <property name="position">2</property>
+            <property name="position">1</property>
           </packing>
         </child>
       </object>