Allow simultaneous files open for transcription

[DeniseColbert]

A great many registers have a mixture of all three event types on the images, so this is a large issue, affecting all transcribers.

In WinREG, transcribers open the program three times, one for each event type. In a spreadsheet, you open three separate tabs in the file, one for each event type.

This is a problem in FreeComETT: a user can't be logged in on more than one tab.

Potential solutions/workarounds:

1. Duplicate tab, back button, create new event type file from same package (browser-based duplication of login)
2. Allow users to log in on multiple tabs and have three files in use simultaneously (separate logins)
3. New register type: Composite. Layout changes based on user-reported event types within package

[BevStubbs]

I really like the idea of 3 - A composite register type. Whilst 1 & 2 are mirrored in that you have 3 spreadsheets open at the same time (which is what I use as well). It's more of a problem if you are working on a small screen.

[stsccfr]

Some consequences to consider for this issue: behind the scenes, FreeComETT will be creating and maintaing up to 3 files in this situation, one for each event type. When the user is finished, they will want to upload those files to the server. If verification is being done by file, and if problems are found, then FreeComETT will need to be able to handle that, allowing the transcriber to fix the problems in each file sequentially.