move to QtXml from libxml++
Libxml++ of recent versions requires whole Smartswitch to be built in C++11 mode.
While this is not what we currently want, as many other libraries we use, are built in old C++ mode.
So we want to remove dependancy on libxml++.
I reviewed boost PropertyTree and QtXml.
I found QtXml API to be quite similar to libxml++.
So I decided to switch to QtXml.