fpspreadsheet: Some improvements of spready in relation to merged cells (improved row height calculation for merged block, lots of clean-up, in main form move "show header/gridlines" checkboxes to menu)

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3561 8e941d3f-bd1b-0410-a28a-d453659cc2b4