envmanager: Use overlay for all settings, and cleanup

Devin Linrequested to merge
work/devinlin/envmanager into master

Follow up to !109 (merged)

This incorporates changes from plasma-mobile!729 (merged), and completely removes the legacy approach of writing directly to user config files. Bigscreen specific config settings were also updated, and mobile specific ones were removed.

Also add instructions on running the shell in a window in the readme.

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