fpspreadsheet: Split off common properties and methods from the custom reader/writer classes into a common TsCustomReaderWriter class. Add a logging mechanism for error messages to this common ancestor.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3445 8e941d3f-bd1b-0410-a28a-d453659cc2b4