added alt+scrollwheel -> horizontal scroll

[SwissalpS]

permits reading long lines such as logfiles with more ease

[anzz1]

this is better so you can't scroll left out-of-bounds. idk how to do right and bottom of out-of-bounds tho, while top seems to be handled elsewhere already

function View:on_mouse_wheel(y)
  if self.scrollable then
    if keymap.modkeys['alt'] then
      local nx = self.scroll.to.x + y * -config.mouse_wheel_scroll
      self.scroll.to.x = nx > 0 and nx or 0
    else
      self.scroll.to.y = self.scroll.to.y + y * -config.mouse_wheel_scroll
    end
  end
end