Previously, the property binding created in `Surface.bindIsSplit` would
get freed automatically when the source object (the SplitTree widget)
got finalized. A subsequent call to `bindIsSplit` could then cause a
crash by using the stale pointer to the binding. This bug could e.g. be
triggered by dragging the surface from a single-surface tab to another
tab.
We now create an extra reference to the binding object so that Surface
essentially owns the binding and is responsible for freeing it.
Updated the binding in `SurfaceScrolledWindow` to use the same pattern.
That one was probably fine, because the binding is only created once,
but let's be safe.
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