improve navigation button command

* enable to open a referrer when the button has backward entries of history
* test modifier keys strictly
* select a referrer tab only from visible tabs
refactor constants for UI
add option for style of URL string of tooltip
arrange function parameters into hash
minor fix

* use |let| instead of |var| for a variable, and use |const| for a common
  handler
* rename some things for readability
* keep a decent form of coding
* update comments