[TextEditor] Improved text editor loading performance.
See assistly case #73863. During open a very long line could block the
file open performance noticable long. Now a best guess of the line
length is done & the longest line is determined during line creation.
That should speed up loading by a large margin. OTOH it only works
100% on monospaced fonts, but on non monospaced fonts the editor width
is adjusted during scrolling and it's a fair guess for the initial
horizontal size.