notebook: use the current state to get the padding values

notebook: use the current state to get the padding values

We want to enable the use of different padding values between active and
inactive tabs, so that the two are completely separated (but limited by
the active tab size).
This way themes can decide how bigger the active tab is drawn compared
to the normal one just specifying a different padding value from the
CSS, like this:

.notebook tab {
  padding: 2;
}

.notebook tab:active {
  padding: 4;
}

As a first step, fetch the padding values with the right state flags
from GtkStyleContext.

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