Currently, when a GimpFilter's visibility changes, we rely on its
various visibility-changed signal handlers to rewire the filter
node's graph to reflect the change.  This has two main
disadvantages:

  - There's no easy, generic way to toggle a filter's  effect,
    especially one that is not subclassed, since GimpFilter only
    takes care of the case where visibility becomes FALSE, and does
    nothing by itself when it becomes TRUE again.

  - While GimpDrawable does handle the visibility => TRUE case, it
    doesn't disconnect the filter's input from its mode and
    (potentially) source nodes when it becomes invisible.  As a
    result, while none of the drawable's graph is processed as part
    of the composition when not visible, the mode and source nodes
    do get invalidated when the filter's input is invalidated, such
    as while painting on a layer below the drawable.  This is
    particularly bad for pass-through groups, since their source
    node can be an arbitrarily complex graph, whose invlidation
    incurs a nontrivial overhead.

Instead, don't touch the filter's node at all when visibility
changes, and rather have GimpFilterStack remove it from the graph
entirely when the filter becomes invisible, and re-add it once it
becomes visible again.  This solves both of the above problems, as
well as simplifies the code.