Update submodules: plugins (new commits)

plugins:
    f26e8b3 Chris Smith <chris@dmdirc.com>
            Fix DialogProvider not freeing instances.

            Previously it only released the instance if the window was closed by the user
            interacting with it through the window manager, rather than if it was closed
            programatically (through an OK/Cancel button, for example).

            Gerrit: http://gerrit.dmdirc.com/3091

Change-Id: I65c1ea92f80c28ac193bf2af56ad049453fc708c