Wrong tool window icon size

Found in idea.log
```
2018-07-04 17:13:57,520 [  86208]   WARN - openapi.wm.impl.ToolWindowImpl - ToolWindow icons should be 13x13. Please fix ToolWindow (ID:  ASM) or icon jar:file:/D:/Dateien/.IntelliJIdea2018.1/config/plugins/ASM-BO/lib/ASM-BO.jar!/images/asm.gif
```