<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
### This PR implements various fixes and improvements into
RegistryPreview for saving files:
1. Adds an unsaved file indicator in the title bar like in Notepad. (As
indicator we show the * character before the file title.)
2. The save button behaves like a "save as" button, if the file does not
exist on disk like in Notepad. (Without fix when running as non-admin
you get an access denied error message.)
3. If the app gets closed without saving and the file does not exist on
disk, then the user is now asked for the path. (Fixes crash on clicking
save button while closing the app.)
4. Failed save actions are handled now correctly on dirty closing,
opening files and all other actions that require saving the current
state. They will stop the process.
5. A fix for an incorrect enabled state of the save button after
opening, reloading and saving a file.
6. Reuse file name on save as button, if known. Otherwise use "new file"
template name like in Notepad.
7. Fix an app crash if you click the window's close button a second time
while the "Should save?" dialog is opened.
8. Added an reload dialog in case of unsaved changes.
<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist
- [x] **Closes:** #36876, #36875
- [x] **Communication:** I've discussed this with core contributors
already. If work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end user facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx
<!-- Provide a more detailed description of the PR, other things fixed
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments
<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
Local test build.
---------
Co-authored-by: Gordon Lam (SH) <yeelam@microsoft.com>
