Side Panel OpenOffice/IbreOffice doesn't preserve visible state closed when restarting

[Siedlerchr]

1. Click on Tools-> OpenOffice LibreOffice connection
2. Click on the X in the corner of the panel or again in the menu
3. The panel is closed
4. Restart JabREf
5. The panel is open again

Refs #4341

[Siedlerchr]

I tackled this in my PR