Keep aspect ratio of images if width or height is not known

Created originally on Bitbucket by tfried (Tom Gottfried)

Was already merged in Bitbucket before import, marked as merged by the import user

In case the aspect ratio of images to be filled into a template is not known when creating the template, setting either a fixed height or width for the image might not give the desired result: the resulting image might exceed the dimensions of the placeholder frame. The changes in this PR make keep_ratio being respected also if neither width nor height is set, by having the image scaled to a size that fits into the placeholder frame.