Use qt_add_qml_module for the dice demo.
Make the demo qmllint clean by removing unused imports, unqualified
accesses and more.
Use loadFromModule instead of load, and rename main.qml to Main.qml to
be loadable. Also add a missing qmldir file for the qmake project to
work with loadFromModule, and fix the prefixes in the .pro file.
Pick-to: 6.8
Task-number: QTBUG-100100
Change-Id: I786dcb971980f48e91b0aeaf9d65f285a78c260e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>