Add B_NOT_STACKABLE and B_NOT_TILEABLE flags to BWindow

[jscipione]

Now that Stack & Tile is part of app_server and the default decorator, it would be nice if some flags were added to BWindow to indicate that the window should not be added to a stack and/or not be tiled. This way the developer can opt-out of the feature for a window if it is not appropriate as part of a stack/tile group. I am thinking of preferences windows and windows that have controlled resize limits.