Unify initial waiting of debug services

The debug services generally behave in one of two ways when initializing:
Either they block the initializing thread until some configuration is
passed over the network or they just go on. By introducing a generalized
configurable debug service the various ways of waiting on initialization
are cleaned up. The API defined for it also allows for engine-specific
initialization.

Change-Id: Id5685ef17d2a7eb1222629f7caa5ec53076d47b2
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Add a debug service for controlling qml engines

Like this we can control the starting and stopping of qml engines from
the client without having to extend each of the other debug services.

Change-Id: I5f1c077b6cfa0e628c32e8bcdea2ec053e310509
Reviewed-by: Kai Koehne <kai.koehne@digia.com>