core: consolidate all permissions checking into main D-Bus interface
Moves the system settings permissions checking into the core service's permissions checking, which at the same time enables 3-way permission reporting (yes, no, auth) instead of the old yes/no that we had for system settings permissions before. This allows UI to show a lock icon or such when the user could authenticate to gain the permission. It also moves the wifi-create permissions' namespace to the main namespace (not .settings) since they really should be checked before starting a shared wifi connection, rather than having anything to do with the settings service.
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