Wrong tool window icon size

Found in idea.log
```
2018-07-04 17:13:57,526 [  86214]   WARN - openapi.wm.impl.ToolWindowImpl - ToolWindow icons should be 13x13. Please fix ToolWindow (ID:  Gradle View) or icon jar:file:/C:/Program%20Files/JetBrains/IntelliJ%20IDEA%202018.1.5/lib/icons.jar!/general/add.png
```