Improve path search in configure.

Improve path search in configure.

- Remove duplicated code locateFile/locateFileInPaths.
- Move basic path search functionality to Environment.
- Add functions for headerPaths/libraryPaths to  Environment.
- Use QStandardPaths::findExecutable().
- Replace Environment::detectExecutable by
  QStandardPaths::findExecutable().
- Introduce static path lists in findFile() to avoid
  repeated directory scans

Change-Id: I9b93066a3de65f40527780d6ddf7989bca35cd04
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>