Exporter: better handling of duplicate object names

[MoritzBrueckner]

Fix/Workaround for #1523.

Iron does not allow duplicate names for objects but the now replaced code was still adding exporting objects to collections if their names were already "taken".

Also, objects in collections are now always exported with their full asset name, taking the collection name into account which decreases the rate of name conflicts massively.

In the very rare case that an object from a collection still has the same name as another object in the scene (that object then must be named [collection_name]_[object_name].blend for this problem to occur, which is very unusual), a warning is printed to the console and the collection object with the duplicate name is not exported.

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Just an idea for the future (off-topic): Warnings sometimes get buried amongst all other kinds of messages in the console and tend to get overseen. I think a message at the start of the game that says that n warnings occurred during export could help here. Or a hint in the UI if it is not too annoying). Maybe I will look into that and open another PR.

[luboslenco]

Thanks!

Just an idea for the future (off-topic): Warnings sometimes get buried amongst all other kinds of messages in the console and tend to get overseen. I think a message at the start of the game that says that n warnings occurred during export could help here. Or a hint in the UI if it is not too annoying). Maybe I will look into that and open another PR.

Sounds good. We could also de-clutter console output a bit by default, I think khamake has some silent mode switch.