Doc: replace "element" with "QML type" or "item"

Try to make terminology more consistent, by using
"QML type" when describing the types, "item" when
describing instances of QML types, and "component"
when describing objects built by using QML types.

Change-Id: Ie03f3a7e6f483471e344ae1f35bec0742173b27c
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>