tab -> 2 spaces, remove trailing spaces
use "sdk/" namespace in require
make extension work with the latest add-on sdk

* remove 3rd party libraries for settings button, use
  simple-prefs control instead
* display settings in a panel instead of tab
do not pass any metadata to the settings panel (kiss)
must be a bug of add-on panel that doesn't honor width CSS rule
remove obsolete credits
rebuild xpi against add-on sdk 1.15