Object: Add usage hint to instantiate Object properties in editor

[akien-mga]

Edited after changing the approach used to fix the bug.

Fixes #36372 as Path2D/Path3D's curve property no longer uses a Curve
instance as default value, but instead it gets a (unique) default Curve
instance when created through the editor (CreateDialog).

ClassDB gets a sanity check to ensure that we don't do the same mistake
for other properties in the future, but instead use the dedicated
property usage hint.

Fixes #36372.
Fixes #36650.

Supersedes #36644 and #36656.

[akien-mga]

There's still the option to consider it a bug for this curve property to use an instantiated resource, and thus we could merge #36644 as the proper fix (but losing the current feature that Path2D/Path3D's curve is auto-instantiated in the constructor).

After discussing with @reduz, we agreed that the better fix would be for Path2D/Path3D not to auto-instantiate their curve property, but to instead have a dedicated property usage flag that can be used for Object properties that should be auto-instantiated when created in the editor.

So I've implemented that.

[akien-mga]

I think this new solution is a good approach overall, and it opens the possibility to do the same for more properties, e.g. GPUParticles2D/GPUParticles3D's process_material (to default it to a ParticlesMaterial and thus have particles working out of the box in editor).

Some concerns though:

• Are there other ways to add nodes in the editor apart from using CreateDialog?
• Custom classes should work fine for the Node being added, but what about properties of custom types? These would likely fail I guess.
• Do we break the least surprise principle if nodes added from the editor and from script will behave differently?