[gtkada] tick_label signal

[manuel.op.de.coul at eon-benelux.com]

And I'd like to make a plea for a provision of To_Marshaller
for the "tick_label" profile in Gtk.Extra.Plot in the next version.

There's also a mistake in the spec of that package under procedure
Axis_Use_Custom_Tick_Labels. There is no Axis_Set_Tick_Labels
subprogram, what's meant is a handler for the tick_label signal.

Manuel