Cannot open projects with multiple set files

[sraboy]

This is currently logged as a Trace error and any file after the first returned from Directory.EnumerateFile() is ignored. The user should be prompted to associate each SET file with a sprite so projects can be merged.

This is only important for databases other than SFRAME, which is all we're working on so far.

[sraboy]

See #11 and #37. These will likely be resolved simultaneously as they all have roots in the same issue detailed in #37.

[sraboy]

#11 and #37 are closed but this one will still require some UI work to expose to the user. As it stands now, additional sets (which are just DataSets) are merged to the currently-open set, if any, by default. Obviously, this can be a problem if a standard sprite, like ballista, was edited because now the tables will contain multiple rows for the same sprite.

I'm not sure how granular I'll get with this but I think at least choosing to exclude tables from the merge would be nice.