[shortcuts][pref] Create ability to Add or Remove Shortcuts using Preferences window

As a user, I want to be able to easily add/remove shortcut add-in items using the TreeView editor in the Preferences dialog.

## Acceptance
* [ ] Add items to GUI editor
* [ ] Remove items from GUI editor
* [ ] Save editor results to JSON

## Out of Scope
* Editing JSON data (text and targets)
* Moving items via drag-drop