[th/bridge-default-value-cleanup]

Various cleanup to bridge handling.

I am still unhappy how we handle all the meta data (like which properties exist and their types/default values). For bridges, we partly open code each property (nm-setting-bridge.c and ifcfg writer) or have some duplicated meta data (nm-device-bridge.c and ifcfg reader). This should all be better, not only for bridges, but for all kinds of settings.

Point is, this will probably change in the future again, but for now, I think we should do this.