Fixed issues introduced by upgrading to Qt 5.15.
WIP: upgrade to Qt 6
WIP: QML fixes. [sklp-ci]
WIP: macOS font fix.
WIP: backend is a now a singleton.
WIP: remove version number of import.
WIP: fixed missing Action in qmldir.
WIP: fixed errors on program exit.
WIP: CMake detects host arch on mac if not specified.