kconfig: gconf: inline fill_row() into set_node()

The row[] array is used to prepare data passed to set_node(), but this
indirection is unnecessary. Squash fill_row() into set_node() and call
gtk_tree_store_set() directly.

Also, calling gdk_pixbuf_new_from_xpm_data() for every row is
inefficient. Call it once and store the resulting pixbuf in a global
variable.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>