fpspreadsheet: Improved display of background color in spready. (Note: the unit colorbox.pas is borrowed from the LCL folder, it can be removed once patch #0026707 is included in Lazarus).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3550 8e941d3f-bd1b-0410-a28a-d453659cc2b4