Previously, we used U+FFFC as a marker in the debugger (for deadkeys and
LDML keyboard markers), which displayed as the letters OBJ in a small
dotted square. However, we switched the underlying component to
RichEdit, in order to better support rendering of many scripts (EDIT has
some problems). The RichEdit control silently converts U+FFFC to U+0020.
I switched to U+0001 as this is handled better, but has an unfortunate
missing-glyph marker.
In the future, it would be better to use a non-character marker
(leveraging RichEdit), but this is a much more complex change, and this
fix at least matches previous functionality.
Fixes: #13293
Fixes#10458.
Refactors the LDML keyboard test window significantly, in order to be
able to support markers. Cleans up the state machine for the debug
window in the process, as that was complicating the changes. Removes
a lot of extraneous code in the debug window.
