Fix Bug 181866: Background image should fit the slide/page size

I changed the behaviour to the following:
* If a picture is inserted that is smaller than the page size it will be centered to the page.
* If a picture is inserted that is bigger than the page size and the ratio of the image is nearly the same as the page it will be stretched to the page.

If the ratio differs more the image will be scaled to fit onto the page and will be shown centered.

When the featue and string freeze is lifted I would like to add functionality to modify all these parameters.