Spreadsheet: Add conditional formatting

Currently only supports setting the foregound and the background colours.
This patch also unifies `foreground_color' and `background_color' used
throughout to a `Format' struct, in hopes of getting more formatting
options one day :P

Applications/Spreadsheet/SpreadsheetView.cpp

@@ -140,6 +140,7 @@ SpreadsheetView::SpreadsheetView(Sheet& sheet)
                 auto& cell = m_sheet->ensure(position);
                 cell.set_type(dialog->type());
                 cell.set_type_metadata(dialog->metadata());
+                cell.set_conditional_formats(dialog->conditional_formats());
             }
 
             m_table_view->update();