notebook: allow using different padding values for the active tab state

notebook: allow using different padding values for the active tab state

The code before was basically adding and removing the same padding value
in two different places during the allocation cycle.
Instead, what we want to do is to offset the inactive tab allocation by
the difference with the active tab padding, to ensure the tab content is
always drawn centered and in the right position.

https://bugzilla.gnome.org/show_bug.cgi?id=659777