libtorc-baseui: Add a 'detached' state to UIEffect

- this resets the widget's position so that it is no longer positioned
relative to its parent but relate to the screen/top level window.

- needs to be used with care for widgets that can take focus - as it is
easy to break focus handling (e.g. detaching from group widgets)
libtorc-baseui: Fix UIGroup rendering of children that are not visible
libtorc-baseui: Remove an unnecessary Deselect in UIGroup
libtorc-baseui: Add GroupSelected and GroupDeselected signals

- needs some tweaking as there are cases where transitioning the
selection from one child widget to another still triggers an additional
GroupDeselected/GroupSelected signal pair (with associated impact on
animations)