Make MCE introspectable over D-Bus

Previously mce did not support org.freedesktop.DBus.Introspectable
D-Bus interface which required use of workarounds for making method
calls or listening to signals from qt and qml applications.

Augment mce dbus handler structures to hold XML introspection data too.

Allow registering of dummy signal handlers for the purposes of holding
introspection data for outbound signals.

Unify all dbus handler registration to happen via array-of-handers
functionality and provide introspection data for all method calls
supported by mce and (non-configurable) signals sent by mce.

Do diagnostic logging if mce still sends non-introspectable signals.

Implement org.freedesktop.DBus.Introspectable D-Bus interface for mce.

Allow use of org.freedesktop.DBus.Peer D-Bus interface with mce.

[mce] Make MCE introspectable over D-Bus. Fixes JB#21034
Do not use numeric exit value
Merge pull request #207 from spiiroin/add_introspect_support

Make MCE introspectable over D-Bus