* create ScFlatBoolRowSegments in ScMultiSelIter only if necessary
* create ScMultiSelIter only for affected columns in
  ScTable::MergeSelectionPattern() using ScMarkData::GetMarkedColSpans()
* obtaining a full ScRangeList in ScMarkData::GetMarkedColSpans() was
  completely unnecessary, use existing selection containers instead and also
  if possible avoid creating a segment tree that needs to be converted to
  vector, directly use a vector instead

Improvement:
* under ScDocShell::Load()
  previous: Ir:  26 454 571 612
       now: Ir:  18 811 368 362

* thereof under ScTable::MergeSelectionPattern()
  previous: Ir:   4 104 164 533
       now: Ir:     664 738 808

Change-Id: I95895a3a000c1a0b7895fb0696b0889c6d6b4d49