window: set the default width and height

Since the window can shrink down too much, it makes
sense for us to have a sane default width and height.

720x480 was chosen because that's the default resolution
for CRT TVs.

https://bugzilla.gnome.org/show_bug.cgi?id=766922

shell/window.ui

@@ -4,6 +4,8 @@
   <template class="CcWindow" parent="GtkApplicationWindow">
     <property name="can_focus">False</property>
     <property name="window_position">center</property>
+    <property name="default-width">720</property>
+    <property name="default-height">480</property>
     <signal name="notify::window" handler="gdk_window_set_cb" object="CcWindow" swapped="no" />
     <signal name="map-event" handler="window_map_event_cb" object="CcWindow" swapped="no" />
     <child>