[RegPreview] Various improvements on how files are saved

[htcfreek]

Summary of the Pull Request

Important

• Conflicts with [RegPreview] Init with header and add NEW button #37626 and needs update after this to add the following things to the "NEW" code:
  • Correct handling of save button after loading new file.
  • Stop if save fails.
  • Use new "AskFileName" method.
  • Update "unsaved" indicator.

This PR implements various fixes and improvements into RegistryPreview for saving files:

1. An unsaved file indicator in the title bar like in Notepad.
2. The save button behaves like a "save as" button, if the file does not exist on disk like in Notepad.
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.
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.

PR Checklist

• Closes: [RegistryPreview] Crash clicking save on message when closing #36876, [Registry Preview] Save as behavior on save button for temp file #36875
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Local test build.