gui_mode and mouse_wheel_zoom functions?

[Rik]

2/18/15

John,

These functions seem to have disappeared when you re-worked the pan/zoom
functions.  They still have @DOCSTRING entries in plot.txi which is how I
came upon them.  I suppose the question is whether you want to deprecate
them for 2 release cycles, or whether they are just too difficult to
maintain, in which case we should note the deletion in the NEWS file.

--Rik