Colour the message editor, and let Escape close it

The lines are coloured the way N1MM colours them: grey for a comment, blue for
the keys while running, green for the keys while searching. Anything past the
twenty-four lines that are read is red, so a file with one line too many says so
where the operator is looking rather than quietly losing a message. The colours
are the ones the rest of the program already uses for a verdict.

Avalonia's text box paints one colour, so the colours are painted on a stack of
text blocks, one per line, with the box on top holding transparent text. Both
use the same font and the same padding, so they line up, and they share one
scroll viewer, so they scroll as one.

Three things had to be true for that to work, and each of them cost a run to
find. The lines are rewritten in place rather than rebuilt, because a control
that has been shown once cannot be handed back in a fresh list. The box's own
background is forced transparent in every state through its template, not just
through the property. And the coloured lines sit above the box: a focused text
box paints over what is behind it, which is why the colours were there until the
operator clicked into the text.

Escape closes the editor and keeps what was there before, which is what Escape
does everywhere else in the program.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -102,9 +102,14 @@ to F12 while running, and twelve more give the same keys while searching, with
 the ones left out keeping their running message. The buttons say what the file
 says, and they change as you move between running and searching.
 
+The lines are coloured as N1MM colours them: grey for a comment, blue for the
+keys while running, green for the keys while searching, and red for anything
+past the twenty-four lines that are read, so a file with a line too many says so
+on the screen.
+
 This is N1MM's `.mc` format. **Import…** reads a file somebody published for
 N1MM, **Export…** writes one out, and **Back to the defaults** puts the built-in
-messages back.
+messages back. Escape closes the editor and keeps what was there before.
 
 The macros are N1MM's, spelled the way N1MM's function-key documentation spells
 them, so a `.mc` file written for either program says the same thing. The text