WIP: [th/settings-add2-and-update2-options]

This WIP branch is based on th/settings-delegate-storage branch from !189 (closed).

It adds new D-Bus API:

• add NM_SETTINGS_UPDATE2_FLAG_NO_REAPPLY flag to Update2() (rh#1677070)
• add new AddConnection2 D-Bus API because the existing API was not extensible
• add NM_SETTINGS_ADD_CONNECTION2_FLAG_BLOCK_AUTOCONNECT flag to AddConnection2() to block autoconnect of a new profile (rh#1677068)