Reimplement ScMarkData::GetMarkedRowSpans() to use flat_segment_tree directly.

Change-Id: I90a1d4b3ae2e6aff9a7926b5842bc85ac172683d
Implement GetMarkedColSpans() counterpart.

Change-Id: Ia2f4828a91ad3c89f7867e0f2e0248f885d3fff7
Actually these methods shouldn't be taking a sheet index.

Marked ranges consist of 2-dimensional ranges plus selected sheets.  So the
selected ranges themselves don't care about sheets.

Change-Id: I1c2dfab182282e6b32342b97227b3a7abfaf5179
Remove use of GetMark*Ranges() from DeleteMulti().

Change-Id: I4a17e5f0f1c81c968c821467f326a1c9b06a85a9