Consider deduplicating the the power settings backend, and potentially other pages' backends

[ryanabx]

In cosmic-applets, there is similar power profiles functionality as in cosmic-settings.

See:

https://github.com/pop-os/cosmic-applets/tree/master/cosmic-applet-battery/src/backend https://github.com/pop-os/cosmic-settings/tree/master/cosmic-settings/src/pages/power/backend

I wonder if combining these functionalities into a separate crate would be helpful to reduce code duplication.

I haven't checked the other settings, but this could perhaps encompass network/bluetooth settings, audio, etc. to help ensure that functionality changes get propagated to their respective applets as well as the settings pages.

Perhaps the right place for these would be https://github.com/pop-os/cosmic-settings-subscriptions ?