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

[Yuriy Skalko]

> I cannot even compile everything under clang8
> /usr2/src/lyx/lyx-git/src/support/FileMonitor.cpp:62:9: error: call to 
> 'make_unique' is
> ambiguous return make_unique<FileMonitor>(instance().getGuard(filename));
>                ^~~~~~~~~~~~~~~~~~~~~~~~
> /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/unique_ptr.h:834:5:
> note: candidate function [with _Tp = lyx::support::FileMonitor, _Args =
> <std::shared_ptr<lyx::support::FileMonitorGuard>>] make_unique(_Args&&... 
> __args)
> 
>         Kornel


Does replacing make_unique with lyx::make_unique help?

Yuriy