Previously, the mouse back and forward buttons sent the same codes as
scroll up and down. Now they instead send the same codes as xterm. Mouse
button 10 (in X11 numbering) also now sends the same as xterm, instead
of not sending anything.
This also changes the `send_mouse_event` function which can be called
from kittens to use X11 numbering for mouse buttons instead of what it
previously used, which turns out to be a hybrid of X11 and GLFW. It was
documented to use GLFW numbering, but GLFW doesn't have numbers for
scroll events (that's separate events with x/y offsets) and 4 and 5 in
GLFW is actually back and forward, while `send_mouse_event` interpreted
it as scroll up and down.
That means that this is a breaking change for `send_mouse_event` because
it swaps the number for the middle and right button to be consistent
with X11. I did this because I think it's better to use one consistent
numbering scheme for the function, and because people probably know X11
numbering better than GLFW numbering and GLFW doesn't have numbers for
the scroll buttons.
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