Fix compilation with Unity 2020.3

[Nezz]

UIDocument was introduced in 2021.2, so this PR is needed to make Extenject work with older versions.

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[Nezz]

Actually this doesn't compile with 2021.2 out of the box because the UI Toolkit is a preview package that's not included. Any suggestions for how we could check whether the UI Toolkit is present? I didn't see any relevant precompiler defines.

[sbergen]

Actually this doesn't compile with 2021.2 out of the box because the UI Toolkit is a preview package that's not included. Any suggestions for how we could check whether the UI Toolkit is present? I didn't see any relevant precompiler defines.

Isn't this exactly what version defines are for?

[Nezz]

@sbergen Thanks for the suggestion, this should make it work with all Unity versions. 2021 uses com.unity.modules.uielements, whereas 2020 used the com.unity.ui package.

[Nezz]

@Mathijs-Bakker This should be ready for your review now. I have tested it with:

• Unity 2020.3.19 without UI elements module & package (no UIDocument)
• Unity 2020.3.19 with UI elements module but without package (no UIDocument)
• Unity 2020.3.19 with UI elements module and package (has UIDocument)
• Unity 2021.2.11 without UI elements module (no UIDocument)
• Unity 2021.2.11 with UI elements module (has UIDocument)

[Mathijs-Bakker]

OK! Merged!
Thanks!