More text editor work

[jernestmyers]

Extends the work from #27 that enabled basic CLI product editing in a Textual GUI to include metadata edits and source edits. I implemented tabs in the Textual GUI, one for general product info, one for sources, and another for metadata. The layout gets wonky depending on the number of metadata fields or sources, but I'm not sure there's much to be done about it aside from the user increasing the size of their terminal.

Todos/considerations:

• At this point, I've yet to implement any "primary source" logic for the product sources
• Should we try to add more tooltips to better explain certain inputs, like the one used for existing source filenames? For instance, I imagine we may want to inform the user of the types of different metadata fields.