If a document has a file name associated with it then the prefix is
composed from path components (subdirectories), starting from the one
where the file is located and going up the parents until the resulting
name becomes unique among other open documents.

If a document doesn't have an associated file name, then a sequential
number (starting from 1) is appended to the display name of the
document.

This feature is useful when working with big projects that have lots
of idendical file names across different subdirectories (e.g.
Makefile.in, main.cpp, etc.) that need to be edited at the same
time. It allows to easily recognize such a file when switching
between documents in the editor, w/o the need to place the
mouse pointer over the name entry to get its full path.

Started-by: Dmitriy Kuminov <coding@dmik.org>
Task-number: QTCREATORBUG-10185
Change-Id: I633ea6d9b9b4fce8b67335dbcce1bda29254efde
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>