event-widget : hide hour label on full day event

All events in month-popover displays hour-label indicating their start time,
but all-day events should not display their start time.

This is caused by gtk_widget_show_all recursively setting the events list-box
and all its childrens visible.

We replace gtk_widget_show_all with gtk_widget_show to show the events list-box
and its childrens.

https://gitlab.gnome.org/GNOME/gnome-calendar/issues/250

data/ui/event-widget.ui

@@ -52,6 +52,7 @@
               <object class="GtkLabel" id="hour_label">
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
+                <property name="no-show-all">True</property>
                 <style>
                   <class name="dim-label" />
                 </style>