Improvements in "highlighting", round 2

Continuation to #1214.
Well, this is straight and easy to merge. It adresses:

 - [x] Bugs in the cleanup made to `mark-selection` (4388df166fa5c7e0535d41292f4bbf97324a8d13).
   See 7f564d5 for details about the bugs.
 - [x] Let the [activeline](http://codemirror.net/demo/activeline.html) demo have
   its own module!
 - [x] Improve `active-line`: attach/detach correctly, add a class
   to the `wrap` element so that *any* element of the line can be styled
   (gutters, widgets, code, etc.) and refactored the code a bit.
 - [x] Talk about the new modules in the manual.