Add support for modifying a style.

Up to now it was only possible to modify the name, the description and
remove some iop from the style. It is now possible to add iop from
the selected image or to update it with the setting from the selected
image.
Minor code clean-up.
Use glong instead of long int for num in the gtk list store.

Indeed a G_TYPE_LONG corresponds to a glong which in turns corresponds
to a long int. But seems clearer to use glong as it emphasises the fact
that we are manipulating an item from a Gtk list store.
Use separate section for iop from selected image in style dialog.

iop coming from the selected image are now in a separate section. This
is less confusing and makes the dialog cleaner.
Merge branch 'update-style'

* update-style:
  Use separate section for iop from selected image in style dialog.
  Use glong instead of long int for num in the gtk list store.
  Minor code clean-up.
  Add support for modifying a style.