Extract 2D debug collision drawing from CollisionShape2D to CollisionObject2D

[rburing]

This is more flexible, as it allows displaying collision shapes which were created directly in the physics server without a CollisionShape2D node. In 3D the CollisionObject3D also handles the debug drawing.

Fixes #36499.

Would appreciate testing to confirm that the shapes are still drawn in the right position in all cases.

Note: This change implies that the debug shape of a "lone" CollisionShape2D node (not a child of any node derived from CollisionObject2D) is no longer drawn. I consider this an improvement, as it is less confusing (there is already an in-editor configuration warning saying a CollisionShape2D should only be used as a child of a node derived from CollisionObject2D). Moreover, this is the same behavior as it is currently in 3D.

Note 2: This change also allows to hide disabled collision shapes completely, which was not possible before due to the coupling with the editor code. I've hidden the disabled collision shapes in this PR, but this decision is up for debate (whether they should always be hidden, always be shown greyed out as in the editor, or whether an option to toggle the behavior should be added). In 3D, disabled collision shapes are always hidden.

[KoBeWi]

Aren't server-created collision shapes usually used with server-created physics bodies? In that case they still wouldn't be drawn.

The solution is simple enough to be ok, but the use-case is weird 🤔

[rburing]

Putting the drawing code somewhere deep so it can draw all the things would be even better, but this PR at least puts the 2D situation on par with 3D.

Edit: Still have to make any changes to the shapes (e.g. the changed signal of the shape resources) trigger an update(); marking as draft until that's done.

[rburing]

The editor still has to draw each individual collision shape (even if it is not a child of a CollisionObject2D) to keep editing usable, so I concluded it's not practical to remove the drawing code from CollisionShape2D. I've changed the drawing code of CollisionShape2D to run only in the editor, and the new debug drawing code for CollisionObject2D (based on the CollisionShape2D code and inspired by the 3D code) now runs only when not in the editor and the Visible Collision Shapes option is activated.