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

[Yuriy Skalko]

> Hi Yuriy,
> 
> I’m seeing a crash after this commit when using File->Open recent.
> 
> 
> I’m having 5 files in list and the first entry has nullptr as private data.

Hi Stephan,
Sorry for late answer. Does it crash always on choosing the first file 
in Recent list? I cannot reproduce this on Windows and Linux (using GCC 
in both cases) and suspect it is Clang-only issue.

As I understand Scott used Clang on Linux. Is this crash reproducible there?


Yuriy