Remove GlEventView type

The GlEventViewDetail type now derives from GtkPopover instead of
GtkBin.Hence, The mode setting operation provided by GlEventView type
(which is derived from GtkStack) is no longer needed as a GtkStack
is not required to switch between GlEventViewList and GlEventViewDetail.

https://bugzilla.gnome.org/show_bug.cgi?id=709294