views: sync header labels' style

The Week view exposed a bug where we had different
CSS selectors for the exact same label. This made
them look inconsistent, with different colors and
paddings.

Fix that by ensuring they all use the same style
class. The Year view drawing code needed to be
reworked a little.

https://bugzilla.gnome.org/show_bug.cgi?id=778391

data/ui/week-header.ui

@@ -17,7 +17,7 @@
             <property name="label">Month</property>
             <property name="yalign">0</property>
             <style>
-              <class name="month-name" />
+              <class name="start-header" />
             </style>
           </object>
         </child>
@@ -29,7 +29,7 @@
             <property name="yalign">0</property>
             <property name="label">Week</property>
             <style>
-              <class name="week-number" />
+              <class name="end-header" />
             </style>
           </object>
         </child>
@@ -39,7 +39,7 @@
             <property name="label">Year</property>
             <property name="yalign">0</property>
             <style>
-              <class name="week-number" />
+              <class name="end-header" />
             </style>
           </object>
         </child>