RFC: cmake build

I've got a bit tired with current build system, because it sometimes doesn't recompile needed files (it doesn't moc correctly headers when Qt version is changed), sometimes it compiles too much (for example it recompiles qrc generated files on every change in conf.pri).

So I decided to write a small CMakeLists.txt for local use to get a stable and predictable build. It is incomplet and inkorrect and doesn't have any option that are present in our current configure, but it already allowed me to compile&link working qbittorrent with clang with enabled sanitizers (the thing I failed to do in qmake).

@sledgehammer999, @Gelmir, @glassez Is there any interest in cmake build? Perhaps if there is enough interest in it, we could add it side-by-side with our current build scripts.