Implemented "Add Existing Directory..." feature.

This feature lets add files from selected directory and its
subdirectories to project. Files to add are selected based on filter
supplied by a user.

Task-number: QTCREATORBUG-9081
Change-Id: I978e87c24c5aeffc4eb74160cd6f4f20096de017
Reviewed-by: Daniel Teske <daniel.teske@digia.com>