Prefix duplicate names in Open Documents to make them unique.

If a project contains many files with similar names that need to be edited simultaneously (e.g. Makefile.in, main.cpp) it will be hard to switch between them in the editor since they all will look the same in the Open Documents window. More details about this (with pictures) and the suggested solution are here: http://coding.dmik.org/post/qt-creator-get-rid-of-duplicates-in-open-documents-views.

The patch is already on Gerrit: https://codereview.qt-project.org/65874. (I was expecting that filling the Changes: field with the proper Change-Id will link to that but apparently not).