added support for pasting shape at mouse cursor position

there are now two alternative ways of positioning the shapes when
pasting them to the document. the first (old) way is to use a user
defined paste offset wich gets applied to the shapes position before
adding them to the document. the second (new) way is to determine the
position of the mouse cursor on the active canvas and paste the shapes
so that the center of the pasted shapes bounding box is at the mouse
position.