Can't select ScriptableObjects assets in LinkFields.

Steps to reproduce:

1. Create an action taking a ScriptableObject and add it to the graph.

2. Click the link button on the ScriptableObject link field for the action.

Actual results: 

Missing Assets tab to allow linking directly to an asset.

Expected results: 

Having an Assets tab to allow linking directly to an asset.

Reproducible with versions: 1.0.5

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): Windows

Notes:

• Brought up in this discussion https://discussions.unity.com/t/support-for-custom-types-that-are-not-monobehaviours-or-scriptableobjects/1552467/3