Set browser.search.openintab to true. Polishes.

 - Safeguard against repatching caused by reenabling
 - Prompt the user on disable/uninstall about browser.search.openintab
 - Get newtab url from Prefservice instead of hardcoding about:newtab
Added icon and license header. Clean up.

- Unregister window observer when addon is disabled.
- Added packaged xpi