Less severe feedback in case of an unknown GUI config parameter

[stefanhahmann]

Description:

When opening the following GUI config, the user gets the following error:

• gui.zip

• "Unknown GUI config parameter: TableSelectionOnly found in GUI file."

• 

The reason for this is that the GUI config was saved with an older version of Mastodon where this parameter existed and now it does not exist anymore. It seems, that no problems are caused by this. However, unexperienced users may think the error message shown to the users indicates an actual error.

Possible solution:

• Exclude this legacy GUI param from leading to this error message, i.e. just do nothing, when it occurs
• Print the message to System.out instead of printing to System.err to indicate that this not actually an error.

[tinevez]

I would be in favor of the System.out solution. As we are still actively developing and changing things the message is useful.