[LyX/master] Add move constructor and move assignment operator for FileName class

[Yuriy Skalko]

> Should it work w/o -std=c++17? What do expect to happen with different 
> compiler switches for build of Qt-libs and LyX?

On GCC it works now with all standards: from C++11 to C++20.

> The show stopper with -mmacosx-version-min=10.10 is:
> 
> /Users/stephan/git/lyx/src/insets/ExternalTransforms.cpp:334:20: error: 
> 'any_cast<std::__1::function<std::__1::unique_ptr<const 
> lyx::external::TransformCommand, std::__1::default_delete<const
>       lyx::external::TransformCommand> > (lyx::external::RotationData)> >' is 
> unavailable: introduced in macOS 10.14
>         Factory factory = any_cast<Factory>(any_factory);

Now for C++17 we are using `any` and `any_cast` implementations from 
standard library (instead of boost's for earlier standards). Seems like 
it is somehow broken on macOS.

Try to leave only the boost conditional compilation branch in support/any.h:


#include <boost/any.hpp>

namespace lyx {
using boost::any;
using boost::any_cast;
}


Yuriy