Improve special key handling in completer
set <select> width to auto
Add COMMAND keycode
Merge pull request #3189 from juhasch/improve-completer

fix notebook completer redraw on metakey press

- set <select> tag to style="width: auto" to fit complete autocomplete string in drop down menu
- handle special keys better, so shift, ctrl, pgup, pgdown et al work as expected in the drop down list (i.e. pgup/pgdown/home/end move selection, shift, ctrl, caps lock do not restart the drop down menu)