Glade has been used to change the .ui definition of the Preference
dialog in commit f2061e94

It has removed the "border-width" property, resulting in a 2px
padding being added around the notebook widget, which wasn not
intended and looks bad.

This issue will be gone in GTK+4, but we need to explicitly set
this style property to 0px in the meantime.

Do it in the code instead, where Glade cannot erase it.

Closes #563