PY-3369 Show different icons for functions and methods

Actual function icon is taken from database-openapi.

python/gen/icons/PythonIcons.java

@@ -47,6 +47,7 @@ public class PythonIcons {
 
     }
     public static final Icon Dotnet = load("/icons/com/jetbrains/python/dotnet.png"); // 16x16
+    public static final Icon Function = load("/icons/com/jetbrains/python/function.png"); // 16x16
     public static final Icon InterpreterGear = load("/icons/com/jetbrains/python/interpreterGear.png"); // 16x16
     public static final Icon IpythonNotebook = load("/icons/com/jetbrains/python/IpythonNotebook.png"); // 16x16
     public static final Icon Jython = load("/icons/com/jetbrains/python/jython.png"); // 16x16

python/src/com/jetbrains/python/psi/impl/PyFunctionImpl.java

@@ -131,8 +131,11 @@ public class PyFunctionImpl extends PyBaseElementImpl<PyFunctionStub> implements
         }
         return PlatformIcons.PROPERTY_ICON;
       }
+      if (getContainingClass() != null) {
+        return PlatformIcons.METHOD_ICON;
+      }
     }
-    return PlatformIcons.METHOD_ICON;
+    return PythonIcons.Python.Function;
   }
 
   @Nullable