[invader-edit-qt] Add import/export

[SnowyMouse]

You should be able to export and import between tags.

How this should work is you right click on the combobox of the struct array and you have three options:

• Export
• Export All
• Import (grayed out unless you exported something and the last thing you exported can be imported into here)

Clicking Export exports the current item. Clicking Export All exports all items. If you use either export option, then the resulting data is stored in an intermediate buffer. Clicking Import imports data that was exported, and this works as if you clicked "Add" to add each one and typed in the data, yourself.

If indices can be checked by name, then they should be checked by name rather than index. This has the advantage that, when you import data, everything will, in theory, be valid, saving the user a lot of time.

Exported data should not be saved to a file. This does have some pros and cons, but it's overall worth it. Pros are that you don't have intermediate files littering your filesystem, and it's fast because you don't need to deal with file dialogs. However, the cons are that you cannot (currently) transfer data between tags of the same path in different tag directories, and you cannot transfer data between two different instances of invader-edit-qt.

Overall, this should accomplish the goal for most people, potentially saving a lot of time.