* src/dialog-cell-format.c (create_background_radio): Activated
the patterned background control (although in my machine the
boxes are all black) and added a radio button to let the
user do nothing with backgrounds (analogous to the one in
foreground section).

* src/dialog-cell-format.c (apply_coloring_format): Now this
functions takes into account the changes in create_background_radio,
that is, if the user decides not to change the background,
the background will not be changed. Also, assigned default
values to some variables, just to avoid an annoying warning.

* src/dialog-cell-format.c (create_coloring_page): If there
are different backgrounds in the range, select by default
the "no change" radio button for the background.