Add optional column parameter to QTreeWidgetItem.setFlags

Checking the current (6.6) Qt docs for QTreeWigetItem.setFlags there is no way to specify flags for specific columns, only the entire row. It would be ideal if there were an optional argument for column so that you could specify which columns are editable for example.

All other `set` methods for this class allow you to specify which column you're setting such as setText, setCheckState, etc. so it makes sense to do this for setFlags.

I see people on stackoverflow and the Qt forums using hacky workarounds to make certain columns in a QTreeWidget editable because a straight-forward way is not provided by the API.