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