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conversation-view: Add hack to force TextView to wrap rather than grow

GtkTextView's horizontal size request will grow over time
when added to a GtkScrolledWindow with horizontal scrolling
disabled, even when the TextView is set up to wrap lines.

As a workaround, we overwrite its size request with a constant
width to force the text to wrap around.

See 65ce0de0e3abc9de4fd672c31a92120bf548c518 commit in polari.