class Fl_Return_Button : public Fl_Button
A return button has a built-in shortcut for the Enter key. Typing the
Enter key generates the callback(). Otherwise it acts like a normal
Fl_Button. It will draw with an arrow to indicate that the Enter key
works.
The shortcut will not work if a Fl_Multiline_Input or other
object that absorbs Enter FL_KEYBOARD events has the focus.
Fl_Return_Button(int x,int y,int w,int h,const char *l);
Constructor.