UI/Frame

One of the features which users are asking for is a sort of border, frame or similar so that they can easily see how large their painting is/will be. It is by far the oldest open issue in our tracker, and we've discussed how to solve/implement it there. This wiki-page is meant to be used for discussing how the UI for controlling this feature should be like.

Afaik, letting the frame be "adjustable" on the canvas by dragging is technically unfeasible (comments maxy?). It would also bring in new UI concepts, which is probably unwanted. So it should probably be a simple window. Question is, how much and what information and should be there?

I think we at least need:
 * Size (width + height)
 * in pixels
 * in a user selectable unit (cm, inch, etc..).
 * This means we also need DPI. As a setting or just fixed at say 300?

Other things one might consider:
 * Preset formats dropdown (A4, A3, etc..) + orientation radiobutton (Portrait/Landscape)
 * Custom formats ability: you can create a certain size and save it as a "preset"
 * Being able to set a preset as the default one, and have it appear on new files

Also, how do we handle positioning?