adg: added adg_scale_factor()

New API to convert scale strings, such as "3:1", to their double
approximation, such as 3 in this case.
adg: added adg_gtk_area_reset()

A new API to reset a canvas bound to an AdgGtkArea widget, that is
centering and rescaling it to fill the allocation space of the widget.
adg: implemented autoscale in AdgCanvas

Added a bunch of API for providing the autoscale feature.
adg_canvas_set_scale{,_valist}() provides a way to specify a list of
scaled to be applied in sequence when adg_canvas_autoscale() is called.
When the first proper scale is found, the scale is applied to the local
map and the drawing is centered.

Actually the APIs are quite bindings unfriendly but that solution to
that problem is postponed when resynchronizing adg-lua.

Closes issue #65: http://dev.entidi.com/p/adg/issues/65/