[backend] libqtdee API should be reworked to make it possible to maintain binary-compatibility

[Aurélien Gâteau]

libqtdee API does not make it easy to maintain binary compatibility. It should be changed to use a pimpl and hide all private members and methods. Since this change will break binary-compatibility one (hopefully) last time, one should take advantage of it to also fix method signatures to pass arguments by const references instead of by value.

More information about maintaining binary compatibility for C++ libraries can be found here: http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C%2B%2B