Add gimp_prefs_box_set_page_scrollable() which does what it says, and
make the tallest pages of the prefs dialog scrollable. This method
allows the dialog's smaller pages to still enforce a minimum height
for the window.
Remove the parameters from gimp_prefs_box_add_page() and instead
hardcode them once in GimpPrefsBox. This way they can also easily be
made configurable in the future.
