Remove grouping behavior from `group` element type

[hofstef]

Currently (v3.0.0), a group treats everything it contains as its children. I.e., if you move the group, you also move everything it contains. While this is useful, it makes working with groups challenging:

• Children can only be removed by dragging them out of the group, not by resizing the group.
• If a group is resized manually, it automatically reverts to a size that makes it fit around all its children.
• Groups within groups and intersecting groups are particularly challenging.

Therefore, I propose the following:

• remove all semantic grouping behavior and turn the group element type into a simple rectangle.
  • It could still be used to visually group other elements, but it would not treat them as children.
  • If you want to move a group and all its children together, you could do so with the "lasso" tool (that's 2 clicks and a mouse drag more) and achieve the same result.
• change a group's visualization from solid line to dashed line. This would make it look more like an annotation (which it is, semantically speaking).