Use real path names for buffer lookup by name of temporary external files.

Use real path names for buffer lookup by name of temporary external files.

In case of path names for external files containing symbolic links the real path
and the logical path name may be different for the same file or directory.
LyX is using QDir::tempPath() to create the path name of the temporary directory.
The Qt implementation is free to return the logical or the real path name here and
it happens to be different for various platforms and versions.
The most stable and clean solution is to use the real path name consistently.

(cherry picked from commit f2f861f017bd598c9e5b72f64e10587cbe1e3405)