Factorise the "should this element be printed" decision into an
utility function, which is used by
1) AbstractReportElementLayoutController
   (in charge of non-formatted values: string, image, OLE object, ...)
   which already obeyed PrintRepeatedValues.
2) TableCellLayoutController
   (in charge of formatted values: dates, numbers, ...)
   which blissfully ignored PrintRepeatedValues,
   but obeyed the display condition.

Rename the inconsistently named
 PrintWhenGroupChange
 PrintWhenGroupChanges
 print-only-when-group-change
 print-when-group-change
to
 PrintWhenGroupChange / print-when-group-change

Change the meaning of "PrintWhenGroupChange" to "override
PrintRepeatedValues in first occurrence in group". Since this feature
never worked under the old semantics, no loss of feature. Since we
change the XML attribute name, no ascending compatibility problem: it
will be reset to its default value.

Pursuant to the new meaning of PrintWhenGroupChange, change its
default to *true*, which is the sane default.

Change-Id: Idbe8e90565a354f70db222d047b3d51eeddbbb9f