Make the proofread tooltip get stuck less in wrong places

* Added a class so that all tooltips can easily be removed
* Delay the showing of the automatically shown tooltip by
  few seconds to avoid mispositionings caused by page still
  rendering.
* Hide the tooltip after some time to avoid it staying
  forever in wrong place if DOM is changed (tipsy uses
  absolute positioning and adds the tipsy element to body)
  in a way that causes the element to move on the page. It
  is not end of the world if the user does not see this
  tooltip, given that it is shown every time a proofread
  view is opened.

Bug: 46875
Change-Id: Ic1fdd0e02251857329303f90e5e156312ee9cf60
Merge "Make the proofread tooltip get stuck less in wrong places"