Add a `Fallback Model`

[Adrihfu]

Same functionality as the fallback material
to make debuging more visible.

When a model is not found, instead of not showing any model at all,
have the option in ProjectSettings to add a fallback.

Only PackedScene and Mesh should be supported, if the fallback is left blank nothing will load.

All model types are imported to Godot as a PackedScene (MDL, GLB, OBJ, etc)
in the current release version (2.2.10), the function _entity_setup in prop_studio.gd is design to instance them.

Although the user can create a PackedScene with a MeshInstance,
the fallback directly grabbing a Mesh would be great for ease of use.

[H2xDev]

Hello, @Adrihfu!
Thank you for the report. Yea i think it makes sense to add fallback model as well. It will be in 2.3.0