--get hides fields with default values

[jedahan]

I'd like to start a discussion on having --get show all values by default, not just changed ones. It will help with discovery and debugging. For example, --get bluetooth will not even give a hint that there is a --get bluetooth.mode if you are on the default mode. This is surprising to me.

Thought of a few improvements around this

1. Mentioning that --get and --get blah doesn't show all the properties (likely a human-readable message to stderr)
2. Default to showing all properties
3. Allow passing in --show-changed or --filter-default if people like the current behavior

If we can agree on what work there is to be done, I'm happy to try making the pull requests.