Improve filename rendering in tabs

[bobbylight]

RText currently labels different files with the same name via incrementing numbers, e.g.

file.txt (1)
file.txt (2)

A better approach is that used by IntelliJ - in this scenario, prefix each file name with the parent folder name:

dir1/file.txt
dir2/file.txt

In the case that the parent folders have the same name, I'm not sure whether it's better to go yet another level up, or use the numbering scheme. We don't want the tabs to get too long. But in theory this will be a pretty rare occurrence.