[KBM] Convert RemapBufferRow to a struct with descriptive field names

[masaru-iritani]

Summary of the Pull Request

This refactoring pull request changes RemapBufferRow from std::pair to a struct.

PR Checklist

• A part of : [KBM] Separate function for key press vs key hold (a la dual-key-remap) #5679
• 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
• N/A Localization: All end user facing strings can be localized
• N/A Dev docs: Added/updated
• N/A 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
• N/A 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

Using std::pair has two problems.

• It's difficult to understand what first and second mean without looking into their usages.
• It cannot add more data such as the condition information necessary for [KBM] Separate function for key press vs key hold (a la dual-key-remap) #5679.

Converting the type to a struct with more descriptive field names will make it easier to understand and extend (for #5679).

Validation Steps Performed

• All unit tests pass locally.
• Keyboard Manager Settings can show existing key remaps and shortcut remaps.