Consider using CGenericDBusInterface::relayParentSignals

[oktal3700]

Regarding comment in blackmisc/genericdbusinterface.h:

//! \deprecated KB 2018-08 this seems to be not used anymore and might be removed
void relayParentSignals()
{
    Q_ASSERT_X(false, Q_FUNC_INFO, "crosscheck if still used");

We could use relayParentSignals to avoid a lot of difficult-to-maintain, difficult-to-test boilerplate code in the relaySignals method of all our DBus proxy classes.

The entirety of each relaySignals method of the contexts could be replaced with a call to m_dBusInterface->relayParentSignals(). And the same for the equivalent parts in the constructors of the xswiftbus proxy classes in the XP driver.