date-selector: turn into a entry

After making the date selector use the brand
new date chooser widget, it is now time to make
it an entry.

This commit, then turns GcalDateSelector into a
GtkEntry subclass, and add all the plumbing that
is necessary to make it work as such.

As a side effect, this commit adds rudimentary
natural language support for parsing dates.

data/ui/date-selector.ui

@@ -1,14 +1,12 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
   <requires lib="gtk+" version="3.12"/>
-  <template class="GcalDateSelector" parent="GtkMenuButton">
-    <property name="use_popover">True</property>
-    <property name="popover">date_selector_popover</property>
-    <child>
-      <object class="GtkLabel" id="date_label">
-        <property name="visible">True</property>
-      </object>
-    </child>
+  <template class="GcalDateSelector" parent="GtkEntry">
+    <property name="visible">True</property>
+    <property name="width-chars">16</property>
+    <property name="max-width-chars">16</property>
+    <property name="secondary-icon-name">pan-down-symbolic</property>
+    <signal name="icon-press" handler="icon_pressed_cb" object="GcalDateSelector" swapped="no"/>
   </template>
   <object class="GtkPopover" id="date_selector_popover">
     <property name="position">bottom</property>