Overwriting data and resetting data

[ghost]

At the moment Data Curator handles new vs old data by just opening new tabs whenever new data comes in.
This can be awkward for importing a Data Package which will now have tabs (those that existed before the import) that aren't relevant. This includes the scenario at the beginning on first opening Data Curator as it always opens with 1 tab - see #1020
As Data Curator is starting to offer more flexibility with metadata without data (#987, #986, #852), we need to offer options for DC behaviour (but still using convention over configuration ie: have defaults in place)
The preferences menu is an easy go-to to use to begin with. I'm also mindful not to create too many options or at least show some sensible defaults.
So as a start something like:

• a preferences toggle with "Use/Overwrite existing data on import":
  • an import of data and metadata would reset everything first,
  • for import of metadata only, existing tables and data would be incorporated
  • this might mean we have some more specific preference to reset both data and metadata on import metadata only, so this way we have a kindofa sliding scale of passive to most aggressive options for new data/metadata. Though see below for prompt dialogs (I haven't seen the issue raised yet that users want to dispense with the existing prompt on exits, so no need to over-do the configuration by offering a most aggressive option yet of overwrite without prompts)
  • for import of metadata only, perhaps? new tabs would open to fill the required number of table resources.
  • locking properties would perhaps need to be consistent here across package, table, columns. There might need to be a preference here toggled for Lock/Don't lock on import metadata only. At the moment, Data Curator locks on import Column properties (but user can click unlock after opening) - this seems like a sensible default to begin with (before offering preference to lock/not lock)
• prompt user that there is existing data when importing with, "Proceed? OK/cancel" to allow user to go back and reset or go back and re-save
• offer a 'reset' button (maybe only in Toolbars menu? but where?) that clears all existing data and metadata, with a prompt, "Proceed? OK/cancel" when 'reset' is clicked

Then if the preference "Use/Overwrite existing data on import" is not ticked/selected, Data Curator would act as it does now:

• opening new tabs (no matter what existing data is)

For both overwrite and not overwrite, not sure whether metadata is:

• always reset and overwritten (this makes sense to me for all cases)
• if user selects "Don't overwrite existing data on import", does that mean metadata properties are just merged?. I'm thinking no as I don't think a use case like this exists (although I've been wrong before :P )

OK the above is a bit messy, but I'll spin these out into separate issues as the capacity to work on these is offered.