Add colour depth, transparency options and other setting options in the existing bitmap export and create copy dialogs

In many cases for web publishing colour depth and transparency options are lacking in today Inkscape 0.47 and 0.48 bitmap export and create bitmap copy dialog boxes. Using the Gimp or any other bitmap editor for this purpose only is a waste of time. I propose referring to Xara's dialog box which in my mind is a good example of implementation of these features.
Additionally having the possibility to reduce the number of colours while creating a bitmap copy of an object could allow to create a lot of interesting visual effects.