fpspreadsheet: Reading of xlsx theme colors working (writing is not planned). xlsx reader now creates a default color palette because some xlsx files refer to colors by index although they don't contain a palette.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3442 8e941d3f-bd1b-0410-a28a-d453659cc2b4