Doc: Made \since commands in src/qml consistent.

-QDoc displays the Qt version in the HTML output.
-If there is only one argument to \since, QDoc assumes it is the Qt
 version.

"QtQuick 2.1" -> "Qt 5.1"
"QtQml 2.1" -> "Qt 5.1"
"QtQuick 1.1" -> "Qt 4.8"

Task-number: QTBUG-32172
Change-Id: I3c0da4dce8e44d717a6840bea250b4f0f9d78128
Reviewed-by: Martin Smith <martin.smith@digia.com>