adg: skip AdgCanvas:_adg_page_setup doc

The use of the "_adg_page_setup" keyed data is an implementation detail
that must not be known outside of the adg canvas.
adg: improve adg_canvas_set_page_setup doc

The old documentation was a bit misleading. Improved description and
provided a better example.
adg: avoid double GTK check in AdgCanvas

Minor code refactoring: include gtk.h in under the same preprocessor
branch, avoiding a double #ifdef check.
adg: consider paddings when autoscaling

adg_canvas_autoscale() must take into account the paddings, so the
distance between the drawing and the border can be always customized.
adg: correct margin handling

When the canvas is bound to a page setup, either because of
adg_canvas_set_page_setup() or adg_canvas_set_paper(), keep the paper
size constant, i.e. decrease or increase the *page* size to compensate
increases or decreases of the relevant margins.

Added more tests to check the margins behavior.

Closes issue #109: http://dev.entidi.com/p/adg/issues/109/