Fix bug 5946: Infinite loop when opening file with two files opened with the same...

Fix bug 5946: Infinite loop when opening file with two files opened with the same name

This simply changes the appearance of the display path (in the tab header) in one detail: if the file suffix is not "lyx" (but "lyx15" etc.), the file is displayed with the extension. This fixes the problem (which is due to identical display paths), and it adds, IMHO, useful information to the GUI.

Patch reviewed by vfr.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30977 a592a061-630c-0410-9148-cb99ea01b6c8