Allow files in diff views to be dragged outside of the app

[zwaldowski]

Modernizes the table drag/drop in diff file views so it can pass file names, URLs, and promises to drags that finish outside the app.

I often find myself in the situation, particularly when resolving conflicts, that I want to open a file in an editor that would not be opened by clicking “Open with Default Editor”. This supports that use case and many more.

Dragging a file or files from…	…to…	…will…
any diff file list	another file list in the same view	stage/unstage/split/etc.
any diff file list	any other file list	do nothing.
the working copy	an icon in the macOS Dock	try to open them in the app.
the history	an icon in the macOS Dock	open a copy of the blobs in the app.
any diff file list	Terminal	paste relative paths to the files.
and diff file list	Terminal, with ⌥ held	paste absolute paths to the files.

⚠️ Note, the screen recording shows Dark Mode but this PR does not include #532, it was just recorded from a combined branch.

[zwaldowski]

Updated.

[lucasderraugh]

This all seems like an improvement to me. The only thing I notice is that dragging to Terminal from either the working directory, the index, or history all seems to result in a relative path from the directory level, never an absolute path as your chart says. Looks great other than that.

[zwaldowski]

Good call-out. That might be fine (since it's not a regression), the primary intent here is definitely other apps, but I'll double-check it.

[zwaldowski]

OK! This was my typo in the table. Terminal does no special adjudication for the pasteboard; it always requests strings — thus choosing the relative path — unless Opt is held during the drag. This is fine for now. I think we could get away with sending just the file URL all the time, but in the spirit of not having any regressions I'm leaving the PR as it is. I've updated the table in the PR description accordingly.