Sketcher: Element Widget enhancement: Support for external geometry

Python naming used is "ExternalEdge" for consistency with the name appearing in the status bar when selecting that object.

As discussed:
http://forum.freecadweb.org/viewtopic.php?f=10&t=6861&sid=bd9ad10357f8a7de146c4fbd2d53c23b&start=90
issue #1573 remove unavailable headers
+ share GL widgets when creating further 3d views of a document
+ fix build failure
issue #1788 SVG output of trimmed curves
fixes #1706 rename anaglyph stereo mode red/cyan
move exportDRAWEXE.py to Sandbox WB