system-update: show series in channel selector, prevent downgrade

Because UI extracts and shows the stability part of the channel only, it can present e.g. 2 RC channels for 16.04 and 20.04. Add this information in the UI to clear up confusion.

Also, try to prevent series downgrading from the UI. This is not a supported scenario and can lead to a data loss. Advanced users can still switch between channels of different series via command line or installer.

And while we're at it, fixes an issue where the first channel is highlighted when the device is on a hidden channel and no channel in the UI matched the current one.

Fixes: #3 (closed)

(CC: @cibersheep)

Settings menu	Channel Selector