Send bedSize to 3dviewer

Summary:
For working with D16129

• Create property "bedSize" in 3d View
• Adjust this whenever:
  1. Current Instance connects to a printer
  2. Current Instance changed.

Reviewers: patrickelectric, tcanabrava, laysrodrigues, leandrosantiago

Reviewed By: tcanabrava, laysrodrigues

Subscribers: Atelier: General Development, ervin

Tags: Atelier: General Development

Differential Revision: https://phabricator.kde.org/D16144