Replace broken old utils.debug(...) (since the utils.js → Utils.qml
transition) with a declarative approach:
a console can now be opened with by placing a `DebugConsoleLoader {}`
anywhere.
mainUI has a default console opened by F1/alt-shit-c when no other
console is loaded.
Is now strictly about setting up asyncio/uvloop and providing the
functions for QML to interact with it and call backend coroutines.
The appdirs attribute is moved to Backend.
