Save column state of treewidgets

Unfortunately, it won't quite work automatically, because

• We can't run this in the constructor, since the widget doesn't have data yet
• The widget needs some identifier to use for the configgroup
• Further, some way of applying a default configuration if no configuration was loaded

The caller must thus:

• call restoreColumnLayout manually, after loading the data
• pass some string identifying this view
• (optionally) if the function returns false, manually load the default state