MidoriBrowser gains the show-preferences signal which
gives API users a chance to add preferences.
katze_preferences_add_category now returns a box to
allow freely packing a widget into the page.
The extension panel is packed into the preferences now
rather than being appended as a proper panel.
Motivation is consistency here, such as with Gedit or
Emerillon and also the aspect that extensions often
really do tweak behaviour like preferences do.
Spacing and icons are added for visual consistency.
The goal is to get rid of detailed table setups in favour of
automatic alignment with few hints as well as abstracting away
the exact layout to allow reimplementing it in a different form.
