Add analysis for type passed to CustomPropertyDrawer

[citizenmatt]

The [CustomPropertyDrawer(typeof(...))] attribute sets the target type of the property or decorator drawer class it is being assigned to. There are requirements for the target type, as well as the base class of the custom drawer. Unity will silently ignore if these are set incorrectly (this needs testing).

• The target type must be run time serialisable class, or a class that derives from PropertyAttribute. If not, the drawer does nothing. Testing is required to see if "serialisable class" means only types marked with [Serializable] or also MonoBehaviuour based types.
• If the target type is a serializable type, the drawer type must be a PropertyDrawer, and not a DecoratorDrawer.

[citizenmatt]

E.g.:

using System;
using UnityEngine;

[AttributeUsage(AttributeTargets.Field)]
public class AreaMaskAttribute : PropertyAttribute
{
}

[AttributeUsage(AttributeTargets.Field)]
public class AreaMask2Attribute : Attribute
{
}

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(AreaMaskAttribute))]
[CustomPropertyDrawer(typeof(AreaMask2Attribute))]
public class AreaMaskAttributeDrawer : PropertyDrawer
{
  public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
  {
    EditorGUI.LabelField(position, attribute.GetType().ToString());
  }
}

using UnityEngine;

public class MyBehaviour : MonoBehaviour
{
  // The editor will work for this one
  [AreaMaskAttribute] public int myField;
  // But not for this one
  [AreaMask2Attribute] public int myField;
}