cylc gui: fix gtk warning for toolbar.

GTK versions 2.20+ produce a warning when `cylc gui`
is loaded, due to the use of a `gtk.HBox` in the
toolbar in a `gtk.ToolButton`. This seems OK from
the documentation (and works fine) but it seems that
the GTK code is set up to only be happy with a
`gtk.Misc` object - e.g. a `gtk.Image`.

We used a `gtk.HBox` for the iconwidget in the
layout toolbutton (toggle side-by-side) so that we
could have a left and a right arrow - this now
has to be changed. Given that we can only use one
of these (only a single `gtk.Image`), I've used a
right arrow and moved the button to the View 2
toolbar to make it clearer.

@hjoliver, please review when you have spare time.