We're trying to address multiple issues at once here. The primary one is
that GtkTextView has issues with rendering very long lines, in a way
that causes what are effectively hangs. Meld would also have issues with
highlighting such lines, but we already have guard rails in place for
that.

The solution here is that we have just defined a maximum sensible line
length - 8K, which is quite generous - and we will cancel opening any
file with lines longer than that. While this doesn't directly address
loading binary files, it should indirectly stop them from being an
issue.

The code that we've got does rely on us getting the loading callback
often enough that we think the text view won't have gotten a line that's
*too* much longer than our limit between callbacks, but currently that's
tied to an 8K read size which seems like it's going to be fine for our
purposes.