Doc: Made Qt Quick items' \since consistent.

Qt 5.2:
-"\since QtQuick 2.2" -> "\since 5.2"

Qt 5.1:
-"\since QtQuick 2.1" -> "\since 5.1"
-"\since Qt 5.1" -> "\since 5.1"

Qt 5.0:
-"\since QtQuick 2.0" -> "\since 5.0"

Task-number: QTBUG-32172
Change-Id: I699b00b31373a434ddb8f4caccfef40c588bf8a4
Reviewed-by: Martin Smith <martin.smith@digia.com>