$displayHeader is protected but accessed from outside

While reviewing fd-plugins#6053 (closed), I noticed that $displayHeader is declared protected in simplePlugin but public (using ugly keyword var) in most plugin classes.

And it is accessed from outside, sometimes from simplePlugin classes which should work, but also from rest.php which is gonna be a problem.

It is always accessed to test if tab is active, so we should just add an isActive method and use this.