[content] added self-destruct
Consequences:
- only one instance of the extension can be running at a given moment
Why?
- the extension is updated in firefox and safari without restarting the browser,
  this could lead to two running instances of the content script.
- once in a while I forget to disable the Webstore version or the release version
  when I load the debug version
Why is having multiple instances running bad?
- because both will listen to DOM changes, which will be triggered by the other
  content script. One mention where the script adds a DOM node is enough to create
  a perpetual loop of modifications (script A then B then A then B then A ...)
[content] about tab on own profile is now also processed
[content] use smallest column in about tab
Adds the info pane to the smallest column instead of the last column.