minor fix: many changes to unify forms of comment

* many comments: Capitalize and add period.
* update comments
* fix alignments: add new lines, rearrange function/variable for readability
remove unnecessary |event.stopPropagation|

* add |event.stopPropagation| with |event.preventDefault| just to be sure
make constant for custom keys of element

* separate them from UI setting.
* SiteStyle, URLbarKeyword, MoveTabToWindow: extend a custom property instead
  of a native attribute |menuitem.value| to avoid confusion with others.
support creating HTML element
use <span> for inline styling
remove custom text wrapping and rely upon native one
implement copy selection command
add margin to drag panel
find new string of global findbar on tab selected
use |KeyboardEvent.key| instead of deprecated |keyCode|
rewrite with switch statement for readability
separate workaround for command error status display
tidy if conditions
remove unused pref buttons
handle error for submit mode navigation
update accent strings in URL tooltip
remove extra definition for value only used in one place