fpspreadsheet: Redo internal structure of merged cells (use MergeBase cell instead of neighbor links). 
Suppress TsWorksheetGrid painting of inner grid lines of merged cells.
Fix inserting columns and rows running through merged cells. 
Silence some warnings. 
Fix ods reading merged cells incorrectly.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3569 8e941d3f-bd1b-0410-a28a-d453659cc2b4