The old NMExportedConnection was used for both client and server-side classes,
which was a mistake and made the code very complicated to follow. Additionally,
all PolicyKit operations were synchronous, and PK operations can block for a
long time (ie for user input) before returning, so they need to be async. But
NMExportedConnection and NMSysconfigConnection didn't allow for async PK ops
at all.
Use this opportunity to clean up the mess and create GInterfaces that both
server and client objects implement, so that the connection editor and applet
can operate on generic objects like they did before (using the interfaces) but
can perform specific operations (like async PK verification of callers) depending
on whether they are local or remote or whatever.
