Help: Do not pass zoom factor to constructor

Introduce HelpViewer::setScale instead

Change-Id: I9d50483bc1fa4de89de53db40eda348379bd0ae9
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Help/OS X: Use native viewer by default if welcome uses QQuickWidget

Since using window container breaks the native view hierarchy
(QTBUG-39363), we can use the native viewer only if we use QQuickWidget
for the welcome screen.

Task-number: QTBUG-39363
Change-Id: I26407a5c1db558d9a8ac6d79c37e3ffa0e009ed6
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>