fdo#71669 Poportionally scale images by default in Writer

When holding "shift" at resizing, objects are resized proportional.
With this commit, proportional resizing for images is the default
behavior and "free" resizing is enabled when holding "shift". The
behavior for other objects is unchanged.

Change-Id: I484455a8bd3edb879c75b9a27988cebeb843678b