allow each instance of PyCallableType to tell whether it's actually callable or not; distinguish class definition and class instance for Java class types; correctly treat Java constructors as callable (PY-4269)

python/psi-api/src/com/jetbrains/python/psi/types/PyCallableType.java

@@ -11,6 +11,11 @@ import org.jetbrains.annotations.Nullable;
  * @author yole
  */
 public interface PyCallableType extends PyType {
+  /**
+   * Returns true if the type is callable.
+   */
+  boolean isCallable();
+
   /**
    * Returns the type which is the result of calling an instance of this type.
    *