Add a new location for QML 2.x imports

Add a new location for QML 2.x imports

This commits adds a -qmldir configuration option for the configures to
allow the user to change the default location (it defaults to
$archdatadir/qml).

It adds a QLibraryInfo::Qml2ImportsPath value for
QLibraryInfo::location, a qmake property of QT_INSTALL_QML and a qt.conf
configure location entry "Qml2Imports".

At the same time, it makes the qmake .prf files dealing with QML plugins
be the QML 2 version. Those files are new in Qt 5, so we have the option
to choose which version we want to use.

Discussed-on: http://lists.qt-project.org/pipermail/development/2012-October/007136.html
Change-Id: I8c1c53e8685a5934ed0a9a42ba5663297b81a677
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>