Fix role for generic Accessible items

Currently QQuickItem specifies Pane as default role which at least on
linux leads to big confusion. Instead use QAccessible::Client which is
in line with QWidget.

Change-Id: I06b287843a0bf1546f16e67c53ea5d2321ba214b
Task-number: QTBUG-40136
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>