X11: Add support for horizontal mouse scroll axis input.

src/Fl_x.cxx

@@ -1466,6 +1466,12 @@ int fl_handle(const XEvent& thisevent)
     } else if (xevent.xbutton.button == Button5) {
       Fl::e_dy = +1; // Down
       event = FL_MOUSEWHEEL;
+    } else if (xevent.xbutton.button == 6) {
+      Fl::e_dx = -1; // Left
+      event = FL_MOUSEWHEEL;
+    } else if (xevent.xbutton.button == 7) {
+      Fl::e_dx = +1; // Right
+      event = FL_MOUSEWHEEL;      
     } else {
       Fl::e_state |= (FL_BUTTON1 << (xevent.xbutton.button-1));
       event = FL_PUSH;
@@ -1494,7 +1500,9 @@ int fl_handle(const XEvent& thisevent)
     set_event_xy();
     Fl::e_state &= ~(FL_BUTTON1 << (xevent.xbutton.button-1));
     if (xevent.xbutton.button == Button4 ||
-        xevent.xbutton.button == Button5) return 0;
+        xevent.xbutton.button == Button5 ||
+        xevent.xbutton.button == 6 ||
+        xevent.xbutton.button == 7) return 0;
     event = FL_RELEASE;
 
     fl_xmousewin = window;