Creating a plain Handle() should yield a NULL mpv_handle.
Also, remove the redundant non-const definition of the conversion
operator. At least in this situation it's not needed.
Also, add include guards around qthelper.hpp.
This provides some helper functions and classes for C++/Qt. As the top
of qthelper.hpp says, this is built on top of the client API, and is a
mere helper provided for convenience.
Maybe this should be a separate library, but on the other hand I don't
see much of a point in that. It's also header-only, but C++ people like
such things. This makes it easier for us, because we don't need to care
about ABI compatibility.
The client API doesn't change, but bump it so that those who are using
this header can declare a proper dependency.
