Replace FCmdrSettingsBackend with FCmdrServiceBackend.

Reworking the backend class design.  Having a backend instance per
profile makes it hard to coordinate actions across multiple profiles.
Instead just have one backend instance for each extension, and let
FCmdrService manage the backends.

Case in point: FCmdrGSettingsBackend will need to invoke the "dconf"
command-line tool after updating system database key files and lock
files to have the changes take effect.  But "dconf update" updates
ALL system databases in one fell swoop, so we only want to invoke it
once after all profiles are applied.  The redesigned backend API
makes this easier.