Improve GltfImport constructor API

[atteneder]

Problem

Currently there's a single GltfImport constructor that takes 4 optional parameters.

Problems with that:

• Already quite overloaded, this will be even more so once more parameters get added.
• It's not possible to change the behavior of the logger parameter to create a default ConsoleLogger if null is provided, as that takes away the possibility of not setting a logger.

Solutions

1. Create explicit overloads with no optional paramters at all.
2. Implement a Factory/Builder like pattern with chained setup methods.

Solution 1 might lead to many overloads, potentially conflicting with the original one.

Solution 2 is potentially cleaner.

[atteneder]

Potentially part of #739