Allow UI for multiple files to be linked, for faster switching, or even includes

Since this will require a little bit of a change in the UI, but might lead into a few other features, i'm creating a distinct issue for it.

I think the easiest way to do this, would be to add an expansion tile button on the far right of the file button, that will drop down and show a list of files, maybe in order of most recently selected. Tapping on a file will make it the primary selected file. Maybe swiping on the file will remove it from the list.

Hmm, and since the framework I'm using makes it easy to have a "title" and "subtitle" on the tiles, I think I might add a hashCode of the URI, or some clue to make it easier to distinguish between files. And maybe add back in the info icon I had which would pop up with information like the file's URI, etc., in case users have files with identical names. These platforms and cloud providers don't always include a useful path with the linked file. Maybe I can convert a hashCode to a color, and add a unique circle on far left of tile.

No files, tests, or entry points are named. First locate the file-selection UI and existing linked-file behavior in the Flutter app, then define the interaction details before implementation. Done means users can view multiple linked files, switch the primary selection, and distinguish or remove files as agreed.

Written by the indexing model from the issue text.