Support horizontal scroll events in mouse tracking.

Horizontal scroll events aren't generated by the traditional mouse
wheel, but they can be generated by trackpad gestures, though this
isn't always configured on.

The cross-platform and Windows parts of this patch is due to
Christopher Plewright; I added the GTK support.

(cherry picked from commit 819efc3c21)

terminal/terminal.c

@@ -7223,6 +7223,14 @@ void term_mouse(Terminal *term, Mouse_Button braw, Mouse_Button bcooked,
                 encstate = 0x41;
                 wheel = true;
                 break;
+              case MBT_WHEEL_LEFT:
+                encstate = 0x42;
+                wheel = true;
+                break;
+              case MBT_WHEEL_RIGHT:
+                encstate = 0x43;
+                wheel = true;
+                break;
               case MBT_NOTHING:
                 assert( a == MA_MOVE );
                 encstate = 0x03; // release; no buttons pressed