Doc: Qt Quick Designer UI forms

Since Qt Creator 3.3 and Qt 5.4, Qt Quick Designer
handles .ui.qml files that are similar to the .ui
files of Qt Designer.

Change-Id: I147e82a111ca7d3b806c4c6c94cc73b35a67785b
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>