Improve handling of slider dimensions

[axtimwalde]

Currently, BDV is tailored to 2D or 3D + t and color channels as sources. Often though, we want to simply add sliders or wheels for some dimensions beyond 2D or 3D. The best implementation that is consistent with the current API is most likely a dedicated Source where a subset of the dimensions can be exposed (may be as [Real]Localizable|Positionable) and the source would return the respective hyperslice for getSource(...) and getInterpolatedSource(...).