edit-dialog: show/hide Repeat widgets correctly

Currently, the repeat widgets - repeat_duration stack
and combobox are visible even when 'No Repeat' is
selected in repeat combobox.

This patch ensures that the repeat widgets are hidden
when 'No Repeat' is selected. When not selected,
repeat_duration_combo becomes visible which itself takes
care of the visibilty of the stack.

https://bugzilla.gnome.org/show_bug.cgi?id=782755