Test build: Sketcher constrain knots

[luzpaz]

https://forum.freecadweb.org/viewtopic.php?f=9&t=71130&start=50#p623100

CC @AjinkyaDahale

[AjinkyaDahale]

NOTE: For anyone looking to try the "tangent at knot" constraints, please look at #60 instead, particularly instructions by github-actions like this one: #60 (comment). This PR will remain open as a snap for testing the "position of knot" constraint PR in FreeCAD main or just get closed.

[github-actions]

A build from commit dcb236d has been published to channel edge/pr-56 as revision 310.

Installation instructions

  # Note: not all distros require 'sudo'
  $ sudo snap install --channel=edge/pr-56 freecad

  # If you already have the freecad snap installed
  $ sudo snap refresh --channel=edge/pr-56 freecad 

  # Experimental: Parallel install (see https://github.com/FreeCAD/FreeCAD-snap/blob/master/docs/index.md#parallel-installs)
  $ sudo snap set system experimental.parallel-instances=true
  $ sudo snap install --channel=edge/pr-56 freecad_pr56
  $ freecad_pr56 # Run it

  # To switch back to preferred snap (swap out 'edge' for 'stable' if necessary)
  $ snap refresh freecad --channel=edge freecad

[AjinkyaDahale]

@luzpaz maybe it's time to close this PR? Anybody who wants to test this feature can use #65, and anyone who wants to try individual PRs in master (for whatever reason) can just build them directly.