Use deepbind when loading plugins.

This patch adds the deepbind option when loading plugins. This allows
us to load plugins linked against different library versions, especially
plugins linked against Qt 4. It is not recursive though, and plugins
loading subplugins must still be blacklisted.

Change-Id: Id4f86dc01c1712a08eb4828a852f8a0733eaf899
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>