feat(developer): Add a "Notes" field property for Touch Layout Keys

### Is your feature request related to a problem? Please describe.

This feature request is made to benefit easy and local-ized documentation and serves the universal goals of developer documentation. This may also be analogous to commenting code at the line of code being documented.

### Describe the solution you'd like

The KMD touch layout editor loads a panel of properties and their data on the right side of the application when a key is being edited. The implementation that I would like to see is for this panel to offer a "Notes" field that plain text can be entered into. The text would then be saved as a string in the json body as a "notes" attribute of the key object.

The developer could then record notes on why they defined the key the way that they did, etc. Useful for future reference when updates are made and for someone else trying to understand the work.

A feature like this is available in font editors, like FontLab where notes can be added to any symbol created in the font. This is highly useful for maintaining the font over the long term. Saving notes on a key is a natural analog.

### Describe alternatives you've considered

Documentation recorded externally, which is a pain to locate later.

### Related issues

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### Keyman apps

- [ ] Keyman for Android
- [ ] Keyman for iPhone and iPad
- [ ] Keyman for Linux
- [ ] Keyman for macOS
- [ ] Keyman for Windows
- [X] Keyman Developer
- [ ] KeymanWeb
- [ ] Other - give details at bottom of form

### Keyman version

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### Operating system

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### Device

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### Target application

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### Browser

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### Keyboard name

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### Keyboard version

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### Language name

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### Additional context

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