[AdgGtkArea] Fluent scaling on size allocation

[AdgGtkArea] Fluent scaling on size allocation

While the previous approach was recentering the canvas during a new
allocation, providing a somewhat "scattered" zoom while resizing the
window, this new implementation rescale the canvas without centering it,
hence providing a more fluent user experience.

There is still room for improvements (check the actual
_adg_size_allocate() docblock for further details) but the widget is
basically working properly.

Basically closes issue #3: http://dev.entidi.com/p/adg/issues/3/