Fix IME/CoreTextEditContext not being reset properly

[lhecker]

The first argument to NotifyTextChanged incorrectly was [0,0]
instead of the length of the text to be removed from the
CoreTextEditContext.

Best source of documentation for NotifyTextChanged:
https://docs.microsoft.com/en-us/windows/uwp/design/input/custom-text-input#overriding-text-updates

FYI @DHowett-MSFT (just in case): C++/WinRT uses winrt::param::hstring
for string parameters which intelligently borrows strings. As such you
can simply pass a std::wstring to most WinRT methods without the need
of having to allocate an intermediate hstring. 🙂

Validation Steps Performed

I followed the reproduction instructions of #3706 and #3745 and ensured
the issue doesn't happen anymore.

Closes #3645
Closes #3706
Closes #3745

[lhecker]

BTW the IME integration is still not quite perfect yet. It's truly the easiest to reproduce if you install the chinese language and play around with the editor. For instance if you switch to chinese and press Win. you'll notice that it fails to insert most of the symbols on the "Symbols" tab most of the time, unless you also type some regular words without the use of Win..

[zadjii-msft]

I certainly believe this is better, but considering I don't use the IME on a daily basis, I'll hold my green check pending Phil's approval. Thanks for debugging this!

[DHowett-MSFT]

The winrt::param::hstring thing actually infuriates me a little bit. You cannot generally write code that accepts anything automatically convertible to an hstring because winrt::param is a private namespace. So, if you add an hstring parameter to a function, you must always pass an hstring to it. 😄

[DHowett-MSFT]

I love this. Thank you!

[ghost]

Hello @DHowett-MSFT!

Because this pull request has the AutoMerge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

p.s. you can customize the way I help with merging this pull request, such as holding this pull request until a specific person approves. Simply @mention me (@msftbot) and give me an instruction to get started! Learn more here.

[ghost]

🎉Windows Terminal Preview v0.8.10091.0 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download