Comment 68 for bug 170049

I rewrote the gui code and hopefully fixed all issues, please test.
Use the patch or branch https://code.launchpad.net/~vaifrax/inkscape/bugfix170049

future work:
- for convenience, add buttons to set origin relative to page (see #62)
- for convenience, add draggable origin (see #61, #63)
- use origin as default for new grids
- optionally specify guides relative to this origin