window: Change button order, remove linked

* We shouldn't use `.linked` for header/toolbar buttons,
per https://blogs.gnome.org/alexm/2021/09/10/cleaning-up-header-bars/
* In other apps, back/forward buttons are at the very
beginning of the headerbar.

src/gui/gcal-window.ui

@@ -107,21 +107,6 @@
         <child>
           <object class="GtkHeaderBar" id="header_bar">
             <property name="show-title-buttons">True</property>
-            <child>
-              <object class="GtkButton" id="new_event_button">
-                <property name="icon-name">list-add-symbolic</property>
-                <property name="action-name">win.new-event</property>
-                <property name="tooltip_text" translatable="yes" context="tooltip">Add a new event</property>
-              </object>
-            </child>
-            <child>
-              <object class="GtkButton" id="today_button">
-                <property name="label" translatable="yes">Today</property>
-                <property name="action-name">win.today</property>
-              </object>
-            </child>
-            <child>
-              <object class="GtkBox" id="button_box">
             <child>
               <object class="GtkButton" id="back_button">
                 <property name="icon_name">go-previous-symbolic</property>
@@ -134,9 +119,17 @@
                 <property name="action-name">win.next-date</property>
               </object>
             </child>
-                <style>
+            <child>
-                  <class name="linked"/>
+              <object class="GtkButton" id="today_button">
-                </style>
+                <property name="label" translatable="yes">Today</property>
+                <property name="action-name">win.today</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkButton" id="new_event_button">
+                <property name="icon-name">list-add-symbolic</property>
+                <property name="action-name">win.new-event</property>
+                <property name="tooltip_text" translatable="yes" context="tooltip">Add a new event</property>
               </object>
             </child>
             <child type="title">