Simplify BdvPrompts construction

[xulman]

...by making it always expect an already existing BDV instance (which is passed to BdvPrompts c'tor). This reduces the number
of c'tors -- will remove the single input image vs. two input images flavours, which is anyway business of the client code and not of the BdvPrompts code base. Alongside, getBdv*() could be removed as client must have its own reference on it; it's no longer created in BdvPrompts.

BdvPrompts could offer static utility tools to create BDV instance for the client code -- that's useful for programmers that are not aware of the BdvFunctions utility project.