[Settings] UI feedback of configuration changes

# Description of the new feature/enhancement

Currently, users have to click Save for their changes to be applied, including in the current session.
For most settings, this is counter-intuitive, as the changes cannot be previewed "live" in the application before saving the configuration.

Other Windows applications that share the same user interface design (i.e. WinUI/UWP applications, such as Windows Settings) do not require explicitly saving changes for them to take effect.
The current way Terminal's Settings UI works reflects Win32 applications/settings applets, where OK, Cancel and Apply buttons are present and are required for changes to take effect.

The label of the Save button in Terminal also seems to convey that changes are already applied before saving. It would cause less confusion if it said "Apply & save", to ensure users know that the button needs to be pressed in order for their changes to take effect.

# Proposed technical implementation details (optional)

- Unsaved changes could apply temporarily until the user clicks the Save button. This would be useful to preview certain UI options, such as font & font size, effects...
Closing settings or the application without saving these settings should prompt the user about their unsaved changes.
- Auto-save every change, making the experience feel more seamless.
This approach could come at the cost of the "Discard changes" button, unless there was a new definition of "the changes that were made ".