Added the CHECKBUTTON_PLUS option for the FXCheckButton widget. When this option is set, the check button is drawn as a box with either a plus or minus sign inside to indicate its state: when the check button is "checked" (i.e. its state is TRUE), it is drawn with a minus sign, and when it is unchecked (or the state is FALSE) it is drawn with a plus sign.

Added the getCheckColor() and setCheckColor() member functions. The default check color is the same as the application's foreground color.