This should significantly simplify these panels, by not forcing
them to override GObject.constructed all the time. Most panels
were quite straightfoward.
Make the panel class provide a cancellable that will be cancelled when the panel
is destroyed. Panel implementations can use this and not have to mangage the
cancellable themselves. Consolidate cases where panels had multiple cancellables
that were all being used for this behaviour.
Access strings via pointers, which reduces copying and the chances of leaks.
Simplify cases where string arrays were modified - no need to replace the value as it's no longer allocated.
Always complete the loop so references are freed.
