Add a way for privileged snaps to affect Frame's configuration

[Saviq]

One example is a user request to disable/enable input dynamically (though that sounds like an application level thing):

https://discourse.ubuntu.com/t/ubuntu-frame-how-do-you-disable-the-touch-screen-interface/42026/

Another is display layouts - though this could come from Wayland clients over e.g. wlr-output-management - but that'd require giving control to clients altogether.

One idea we floated around was having / extending the existing content interface to support configuration and/or override files, .d-folder style. Another is gsettings as a potential additional settings backend.