Allow to control the bed dimensions

Summary:
To that effect this patch does the following:

• introduces a BedProperties element carrying the dimensions;
• change GridMesh to have the resolution controlled by a property
• change GridMesh to always be of size 1
• scale the GridMesh to the right dimensions with a Transform

Reviewers: laysrodrigues, tcanabrava, patrickelectric, leandrosantiago, rizzitello

Reviewed By: laysrodrigues, tcanabrava, rizzitello

Differential Revision: https://phabricator.kde.org/D16129