Merge in fixes from other projects that use Utterscroll

Merge in many fixes from my two other projects, debiki-app-play and
Debiki WordPressComments, where I've fixed some Utterscroll issues:

- Don't clear text selection when scrolling (no longer needed, since
  startScrollPerhaps() decides immediately whether to scroll or not, and
  returns `false` if it starts to scroll, so there'll be no new selections).
- Remove hereafter (see above) unneeded functions:
  - dwEnableSelection
  - dwDisableSelection
  - emptyWindowSelection
- Reduce bug risk: add `;` after function declarations.
- Fix recursion depth safety check: start on depth `0`.
- Also change depth limit from 3 to 6, since there was actually no limit at
  all before but everything worked fine nevertheless.
- Don't die on comment nodes, in FF.
Use copyright and license notice suggested by www.gnu.org/.
Add LGPL license files.