Make all dialogs show in the middle of the screen consistently

This is one area that deviates from most usual OS UX. When trying to save a file the modal dialog appears in the middle of the screen. On the other hand when trying to reset to default settings the modal dialog appears right under the mouse cursor. The preferences modal dialog appears in the right upper corner. While not a deal breaker these UI area could use a little polishing an follow consistent UX for each OS. And some of these dialogs could use a little bit of more pronounced contrast against the background.