[KCModuleProxy] Add a new `updateArguments()` dBus method

This method will be called when an application tries to load an already open module, and emits the signal KCModule::argumentsUpdated(). The new argument list is forwarded to the previous instance of the KCM, so it can update its state accordingly.

Also, for QML KCMs, forward the signal to the corresponding ConfigModule::argumentsUpdated()

First commit is just a refactor, keeping the old behavior. Second one adds the functionality.

Requires: kconfigwidgets!26 (closed) and kdeclarative!28 (closed)

Use case is: plasma/kwin!134 (merged)