Use RichEdit's `ITextDocument` interface to retrieve the selection
anchor/caret information, and avoid the side-effects that can arise with
the hacky `EM_GETSEL` / `EM_SETSEL` pattern we used previously.
Changing the selection with `EM_SETSEL` in order to find the anchor
point causes notification messages to be generated that can arrive at an
unexpected time in some text selection scenarios, which ended up with us
having a saved selection in the debugger pointing to the wrong text
range.
The name `Anchor` was a misnomer, because it was actually returning the
caret position, not the anchor for the selection, which is the far side
of the selection from the anchor!
Fixes: #11706
Fixes: KEYMAN-DEVELOPER-18A
Adds breadcrumbs for debug memo to try and narrow down sequence of
events leading to assertion failure with backspace.
Relates-to: #11706
Test-bot: skip
The debugger memo takes input with \n but internally represents this as
\r\n, so we need to strip out \r in order to track caret position
correctly.
Fixes: #13292
Adds support for using RichEdit 4.1, and sets the richedit control into
the correct mode for rendering arbitrary Unicode correctly. Removes
some unnecessary per-character refresh for improved performance.
Fixes: #12454
RichEdit allows text selection one character past end-of-string, so we
need to cater for that as well in passing text ranges to the character
grid.
Fixes: #12454
