This modifies `bindcode` and `bindsym` to use `get_mouse_bindcode` and
`get_mouse_bindsym`, respectively, to parse mouse buttons. Additionally,
the `BINDING_MOUSE` type has been split into `BINDING_MOUSECODE` and
`BINDING_MOUSESYM` to match keys and allow for mouse bindcodes to be
used. Between the two commands, all button syms and codes should be
supported, including x11 axis buttons.