tooltip: re-add `no_offscreen` back to `place`

The tooltip might be partly outside of the screen, and especially the
workarea, e.g. for tooltips on the tasklist.

Calling `awful.placement.no_offscreen` makes sure that it is fully
inside, and will even restrict it to the workarea, not only to the screen.

Closes https://github.com/awesomeWM/awesome/pull/409.

lib/awful/tooltip.lua

@@ -74,6 +74,7 @@ local data = setmetatable({}, { __mode = 'k' })
 -- @tparam tooltip self A tooltip object.
 local function place(self)
     a_placement.next_to_mouse(self.wibox)
+    a_placement.no_offscreen(self.wibox)
 end
 
 -- Place the tooltip under the mouse.