Doc: Modified the term for the Qt global object.

QtQml::Qt is a "QML global object" to differentiate it from the
Qt namespace.

"QML Qt object"
"QML Qt global object"
"Qt global object" (in the context of QML-only pages)

Change-Id: I1d2896ad48e8432c5eb8b18f05af247986bff336
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Doc: Link to QML Qt object in relevant locations

There is no obvious path to find the reference for QML Qt object.
This change adds links to it in Qt QML module index page and QML
Basic Types documentation.

Task-number: QTBUG-31921
Change-Id: I8a4ef1b3e696a0a7dec416d759ae64110e59c6c0
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>