ProjectExplorer: Don't display alien dir separators on Windows

In case you opened more than one project with the same name, it shows
the full path incl. dir separators in the first-level tabs in the
project mode.

Change-Id: Idbf5dfe6ef0c20eecf3a3473011bb2d9b1794cea
Reviewed-by: Orgad Shaneh <orgads@gmail.com>