rename fileName property in prepare scripts to filePath

The fileName property of input/output in prepare scripts actually
represents a file path and is therefore renamed to filePath.
The old fileName property is deprecated but continues to work for a
while.

Task-number: QBS-258
Change-Id: I54524998571557d21f3aaebc0437be168207992e
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
use filePath instead of fileName where applicable

This fixes the recently introduced deprecation warnings.

Task-number: QBS-258
Change-Id: I10660270895ec89a1444d596922c22e9658ca3ab
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>