Snapt to grid for sheet objects

[Emmanuel Pacaud <emmanuel pacaud univ-poitiers fr>]

Hi.

I've just created bug #164869 in which I've attached a patch that
implements a snap to grid mode when moving sheet objects with shift key
pressed.

I'm looking forward your comments about this patch.

        Emmanuel.

http://bugzilla.gnome.org/show_bug.cgi?id=164869