Doc: Adding information about how to run the example.

-includes a file from qtbase/doc/global/includes
-added to example pages (\example)

Task-number: QTBUG-33597
Change-Id: I1356e12826a7702b4431f815d6ec42b28e21003c
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Fix tab visibility regression

Division by 0 caused strange behavior and made the tabs not
get a valid size even when the available size changed later.

Task-number: QTBUG-38294
Change-Id: I456e7a0cc244a76c81832fa580377674bcc5a828
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>