Add mouse buttons to keyboard shortcuts

[georgewsinger]

See #180 for original request. @jfrconley

We're working on a solution now that will allow us to add

  , { _keyCombination = ["KEY_BUTTON_LEFT"]                    , _keyAction = "clickLeft"        }
  , { _keyCombination = ["KEY_BUTTON_RIGHT"]                   , _keyAction = "clickRight"       }
  , { _keyCombination = ["KEY_BUTTON_MIDDLE"]                 , _keyAction = "clickMiddle"       }
  , { _keyCombination = ["KEY_BUTTON_WHEEL_UP"]           , _keyAction = "scrollUp"              }
  , { _keyCombination = ["KEY_BUTTON_WHEEL_DOWN"      , _keyAction = "scrollDown"                }
  , { _keyCombination = ["KEY_BUTTON_XBUTTON1"]            , _keyAction = "...something else..." }
  , { _keyCombination = ["KEY_BUTTON_XBUTTON2"]            , _keyAction = "...something else.."  }

to ./config/config.dhall. The intended use case in this instance is to allow KEY_BUTTON_XBUTTON1 and KEY_BUTTON_XBUTTON2 to send Control +PgUp/PgDn (to switch browser tabs).