Add LPE dialog option to convert Polygons, Polylines, and Lines to Paths.

We don't currently support LPE's on Polygons, Polylines, and Lines, but we don't block adding LPE's to these objects, resulting in crashes. This commit blocks using the LPE dialog to add LPE's to these objects, instead, offering to convert them to Paths.

Fixes #5793 (closed).