Introduce environment variable pointing to shared location of .user files.

Query variable QTC_USER_FILE_PATH for the path and store .user files
there under a tree replicating the project path excluding root.

Use cases:
- Developers working with nightly, clean builds of Qt / Qt Creator.
- Keep the  .user files out of the example directory when creating
  examples for bug reports.

Change-Id: Ib80c5fab3cc943317ec080eba8a6b92c16fc1aed
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>