There are currently 2 different ways to specify what kind of file dialog
should be created: the nDialogType and nFlags ctor parameters.
Simplify that by using the nDialogType for the API specified variety,
and the nFlags only for options that cannot be specified by the API.

This allows to get rid of 3 constructors, and the following constants:
WB_OPEN, WB_SAVEAS, WB_PASSWORD, SFXWB_PASSWORD, SFXWB_SHOWSTYLES