`KeyMapping` was introduced to break up `AppKeyBindings`. `KeyMapping`
records the keybindings from the JSON and lets you query them.
`AppKeyBindings` now just holds a `ShortcutActionDispatcher` to run
actions, and a `KeyMapping` to record/query your existing keybindings.
This refactor allows `KeyMapping` to be moved to the
TerminalSettingsModel, and `ShortcutActionDispatcher` and
`AppKeyBindings` will stay in TerminalApp.
`AppKeyBindings` had to be passed down to a terminal via
`TerminalSettings`. Since each settings object had its own
responsibility to update/create a `TerminalSettings` object, I moved all
of that logic to `TerminalSettings`. This helps with the
TerminalSettingsModel refactor, and makes the construction of
`TerminalSettings` a bit cleaner and more centralized.
## References
#885 - this is all in preparation for the TerminalSettingsModel
## Validation Steps Performed
- [x] Tests passed
- [X] Deployment succeeded
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