remove needless converting to hash
make a utility function to test URL scheme
use arrow function to a callback function
minor fix

* update comments
remove unused import functions
use a reliable method |itemCount| to count menuitems
open search-engine manager as a modal dialog
skip redundant interface test which is done in |PlacesUtils|
use Promise to query Places DB asynchronously
patch |scanPlacesDB| function

* rename to |getPlacesDBResult| to adjust to |promisePlacesDBResult|
* drop a needless portion from try block
catch error of promise
delete script file |URLbarTooltip.uc.js|

* I will create a new script to use keywords for the URL bar later
add new script |URLbarKeyword.uc.js|

* refactored a former script |URLBarTooltip.uc.js|