fpspreadsheet: Make sure that Excel's system colors are kept when colors are added to a palette. Fix BIFF 2 font color mismatch in unit test. All green again.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3471 8e941d3f-bd1b-0410-a28a-d453659cc2b4