[trunk] Add scale factors to img property dlg box (w/ patch)

The purpose of this patch is to add boxes to set horiz. and vert. scaling factors to the property dialogue box for an embedded image. The change is integrated in the undo stack.

The feature has been discussed in the dev mailing list. The rationale for such a feature is to allow consistent scaling between multiple related images and/or precise scaling of idividual images.

The dlg box layout and design can certainly be improved; a (hopefully) working framework is in place, though.

Thanks,

M.