Improve contextual menu generation without separators
and remove comments
Use Gtk.Builder in  IOscopy_GTK_Figure

and add missing signal-list.ui
Fix display of coordinates
Fix key press in graphs, axes/figures enter/leave
Disable button 3 menu.

FIXME: Scroll button not operating. Due to FigureCanvasGTK3Cairo ?
Connect all GTK signals

FIXME: span selector
FIXME: scrollbars
Fixd scrolling with scrollbar

at least it seems for horizontal scrollbar