views/week: Reimplement arrow changes with CSS

src/gui/views/gcal-week-view.ui

@@ -66,6 +66,7 @@
                     <style>
                       <class name="flat" />
                       <class name="circular" />
+                      <class name="week-header-expander" />
                     </style>
                   </object>
                 </child>

src/theme/style.css

@@ -256,6 +256,11 @@ weekgrid > droptarget {
     margin: 2px 0 1px 2px;
 }
 
+button.week-header-expander:checked image {
+    transform: rotateZ(180deg);
+    transition: 200ms;
+}
+
 /*
  * Month cell
  */