Discover Converter capabilities and validate shared binding drafts

designer-preview-api/src/main/java/de/neitzel/fxdesigner/preview/ConverterCapabilities.java

@@ -0,0 +1,15 @@
+package de.neitzel.fxdesigner.preview;
+
+import java.lang.annotation.*;
+
+/** Optional design-time contract. Declare on the concrete Converter; never inherited. */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+public @interface ConverterCapabilities {
+  Class<?> modelType();
+
+  Class<?> controlType();
+
+  /** Whether toAttributeValue is implemented. Forward conversion is always required. */
+  boolean reverse() default false;
+}