Rename QML import from "Qt.WebSockets" to "QtWebSockets"

This is in line with all other modules (e.g. "QtWebEngine 1.x",
"QtQuick 2.x", "QtWebKit 3.x", "QtSensors 5.x")

The old name is retained for compatibility with code from Qt 5.3. This
is achieved by having a dummy qmldir file in the old location.

Change-Id: I66576abe5edb44c3338c47b2980f34574129af01
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>