Devise logic to merge system and user settings

Some layer settings (potentially installed by an admin) may be set in system folders, but the user may want to add layers/settings of their own. There should be logic in the common utility library, which merges these settings.

No files or tests are named. Start by locating the common utility library and the code that reads layer settings from system and user folders. Define how both sources should be combined, then verify that administrator-installed settings remain available while user settings can be added.

Written by the indexing model from the issue text.