week-header: align month_label, week_label and year_label

There was a small alignment issue with week_label, month_label and
year_label in week-view.

Change the y-align property of month_label,week_label and year_label
in week-header.ui to 0, so that they are aligned properly.

https://bugzilla.gnome.org/show_bug.cgi?id=777417

data/ui/week-header.ui

@@ -15,6 +15,7 @@
           <object class="GtkLabel" id="month_label">
             <property name="visible">True</property>
             <property name="label">Month</property>
+            <property name="yalign">0</property>
             <style>
               <class name="month-name" />
             </style>
@@ -25,6 +26,7 @@
             <property name="visible">True</property>
             <property name="hexpand">True</property>
             <property name="xalign">0</property>
+            <property name="yalign">0</property>
             <property name="label">Week</property>
             <style>
               <class name="week-number" />
@@ -35,6 +37,7 @@
           <object class="GtkLabel" id="year_label">
             <property name="visible">True</property>
             <property name="label">Year</property>
+            <property name="yalign">0</property>
             <style>
               <class name="week-number" />
             </style>