This allows to get the version number of XSwiftBus itself. It maintains the same version as swift (e.g. 0.8.7 without the time stamp) and can be used to compare it for compatibility reasons.
ref T394
QtDBus was the main component of xswiftbus' Qt dependency. This is the
first preparation step to get xswiftbus Qt free.
The new implementation is based on the low level libdbus library, which
was also in use by QtDBus itself. But instead of QtDBus, we use now a thin
C++ wrapper. To keep DBus handling async, libevent is used to monitor
timeouts and fds.