## Summary of the Pull Request
When using an _Input Method Editor_ in conhost for East Asian languages, the text cursor is temporarily hidden while the characters are being composed. When the composition is complete, the cursor visibility is meant to be restored, but that doesn't always happen if the IME composition is cancelled. This PR makes sure the cursor visibility is always restored, regardless of how the IME is closed.
## PR Checklist
* [x] Closes#810
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan.
## Detailed Description of the Pull Request / Additional comments
The original implementation hid the cursor whenever `ConsoleImeInfo::WriteCompMessage` was called (which could be multiple times in the course of a composition), and then only restored the visibility when `ConsoleImeInfo::WriteResultMessage` was called. If a composition is cancelled, though, `WriteResultMessage` would never be called, so the cursor visibility wouldn't be restored.
I've now made the `SaveCursorVisibility` and `RestoreCursorVisibility` methods public, so they can instead be called from the `ImeStartComposition` and `ImeEndComposition` functions. This makes sure `RestoreCursorVisibility` is always called, regardless of how the composition ended, and `SaveCursorVisibility` is only called once at the start of the composition (which isn't essential, but seems cleaner to me).
## Validation Steps Performed
I've manually tested opening and closing the IME, both while submitting characters and while cancelling a composition, and in all cases the cursor visibility was correctly restored.
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