install qml files to the same path as target.path

An app with Qt Quick {1,2} Application template is installed under /opt/
and there is no easy way to change the installation path.
This change allows users to set installation path in the main pro file.

Change-Id: I0325c54187f781b1ceaaddfd41e913117fa445e6
Reviewed-by: Kai Koehne <kai.koehne@digia.com>