Load plugins from user config directory too.

Load plugins from user config directory too.

Also adapt the Qt Creator plugin template to make it
possible to choose between deploying locally or into
Qt Creator's build directory.

Change-Id: Ia8566a5034443bc9574a1593eddf0a89eb4d80b2
Rubber-stamped-by: Daniel Molkentin
Reviewed-on: http://codereview.qt-project.org/4178
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>