Added overloads for the FXMessageBox static member functions error(), warning(), question() and information(), to accept a pointer to the FXApp instead of a pointer to an owner window. This allows for the display of free-floating versions of these message boxes.

Added the MBOX_SKIP_SKIPALL_CANCEL option for FXMessageBox, to specify that the message box should have "Skip", "Skip All" and "Cancel" buttons. Along those lines, added two new return values (MBOX_CLICKED_SKIP and MBOX_CLICKED_SKIPALL) for FXMessageBox::execute().