Implement bindsym mouse configuration

Implement bindsym mouse configuration

If a `bindsym` config directive specifies a symbol beginning with
"button", the binding will be given the type B_MOUSE for the indicated
button number.

Example:

bindsym $mod+button2 exec echo 'button two'

This will be interpreted as having input code (now `keycode`) 2 and type
B_MOUSE.

The mechanism to find and run mouse bindings on mouse events is not
implemented.