input: don't map wheel events to BTN_{BACK,FORWARD}

BTN_BACK and BTN_FORWARD are separate buttons. The scroll wheel don't
have any button mappings in libinput/wayland, so make up our own
defines.

This allows us to map them in mouse bindings.

Also expose BTN_WHEEL_{LEFT,RIGHT}. These were already defined, and
used, internally, to handle wheel tilt events. With this, they can
also be used in mouse bindings.

Finally, fix encoding used for BTN_{BACK,FORWARD} when sending mouse
button events to the client application. Before this, they were mapped
to buttons 4/5. But, button 4/5 are for the scroll wheel, and as
mentioned above, BTN_{BACK,FORWARD} are not the same as scroll wheel
"buttons".

Closes #1763

input.c

@@ -2708,7 +2708,7 @@ mouse_scroll(struct seat *seat, int amount, enum wl_pointer_axis axis)
     xassert(term != NULL);
 
     int button = axis == WL_POINTER_AXIS_VERTICAL_SCROLL
-        ? amount < 0 ? BTN_BACK : BTN_FORWARD
+        ? amount < 0 ? BTN_WHEEL_BACK : BTN_WHEEL_FORWARD
         : amount < 0 ? BTN_WHEEL_LEFT : BTN_WHEEL_RIGHT;
     amount = abs(amount);