web, NM model: Run type-specifc hooks at export time.

This will be used to create back references from connections to their
devices, etc.
web: Make master/slave relationship between connections explicit.

And render it.  Mentioning the connection id with every link is
overkill and will be addressed as part of #732.
web, NM model: Multiphase exports.

I knew we are gonna need this...
web, NM Model: invent 'Interface' objects.

To represent network devices that NM knows about but the kernel
doesn't, such as inactive bonds.