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

[Yuriy Skalko]

> The next step from my side should be clang installation and debugging this patch there. But it can happen not very soon. 

It happened sooner than I expected. I've got Clang 11 from winlibs.com 
(it uses libstdc++, unfortunately libc++ is not available here). It 
compiles LyX on Windows without any problems and the opening of the 
recent files doesn't crash. So the problem is definitely in libc++, as 
Jean-Marc suspected from the start.

Yuriy