* src/io-context-priv.h
(enum GnumProgressHelperType, struct GnumProgressHelper): Two extra
helpers (value, workbook) + changes in count helper), removed min_f and
max_f fields.
(struct ProgressRange): New.
(struct _IOContext): Added progress_ranges, progress_min, progress_max.

* src/io-context.h
(enum WbProgressElements): New.
(file_io_progress_set, memory_io_progress_set):
Removed min_f and max_f arguments.
(count_io_progress_set): Added step argument, removed min_f and max_f.
(count_io_progress_update): s/value/inc.
(io_progress_range_push, io_progress_range_pop,
value_io_progress_set, value_io_progress_update,
workbook_io_progress_set, workbook_io_progress_update): New.

* src/io-context.c
(io_context_init): Initialize extra fields: progress_ranges,
progress_min, progress_max.
(io_progress_update): Adjust the value, using current range
<progress_min, progress_max>.
(io_progress_range_push, io_progress_range_pop): New.
(file_io_progress_set, memory_io_progress_set):
Removed min_f and max_f arguments.
(file_io_progress_update, memory_io_progress_update): Don't adjust value
to current range, io_progress_update takes care of this.
(count_io_progress_set): Added "step" argument, initialize extra
struct fields: last, current, step.
(count_io_progress_update): Take value increase as an argument, not
value itself. Return immediately if value hasn't changed enough.
(value_io_progress_set, value_io_progress_update,
workbook_io_progress_set, workbook_io_progress_update): New. Two
additional helpers, "workbook" should be very useful when saving
workbook.

* src/xml-io.h (struct _XmlParseContext): Removed element_counter.

* src/xml-io.c
(xml_read_styles, xml_sheet_read): Don't increment element_counter
manually, just call count_io_progress_update.
(xml_workbook_read): Pass the size of the step to
count_io_progress_set, use io_progress_range_{push,pop}.
(gnumeric_xml_read_workbook): Use io progress helper of type "value".