RFC: Allow enabling Qml debugging at runtime

Normally if you want to enable Qml debugging you need to build with -DQT_QML_DEBUG or put a QQmlDebugEnabler in your code. This is cumbersome to do for every application that one wants to debug. This adds a QQmlDebugEnabler in a central place that can be enabled in Debug mode by setting the environment variable "KDE_ENABLE_QML_DEBUG". This is a union of the combination of the approach taken by plasmashell which hides it only behind an environemt variable and another one seen in other repos to have the compile time definition for debug builds. This is not intended as a final solution but discussion start which one of both approaches we should go with.