globaltoolbar: Use strict === equality in JavaScript

It is used for enumerations (int), counters (also int) and objects. So, it should be safe to change them.

The only case where it is used for strings here, is Qt.platform.os which is confirmed to be of a primitive string type.

(cherry-picked from commit 6970bcc8) (doing it right this time)

Also added comment on showNavigationButtons type and semantics, because it is a mess and has been causing two breakages in past.