fpspreadsheet: Redo handling for formulas: 
- Allow processing of string formulas (conversion to/from rpn formulas, calculation). 
- Drop cell ContentType cctRPNFormula. 
- Drop field RPNFormulaValue of TCell record. 
- Remove all fekXXXX declarations for sheet functions. Function is specified by name now.
- Complete registration mechanism for user-defined formulas.
Adapt all demos
Test cases working
This commit does not yet support: shared formulas, formulas in ods.


git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3506 8e941d3f-bd1b-0410-a28a-d453659cc2b4