More Qt5 compatible Qt6 preparations
Port most remaining QtControls1 context QML items to Controls2.
Add some Qt6 compatibility enhancing ifdefs etc. Also remove unused webengine include, some text codec functions from script bindings that aren't there in Qt6 any more, and other small changes.
Drop AmarokProcess, it doesn't seem to be actually needed anywhere.
Edited by Tuomas Nurmi