Update support for application synchronized updates
In https://gitlab.freedesktop.org/terminal-wg/specifications/-/merge_requests/2, it has been concluded that the original DCS sequence(s) used to enable/disable app synch:ed updates are problematic (not all terminals ignore it).
The proposed alternative is a new DEC private mode, 2026.
DECSET enables app synch:ed updates, and
DECRST disables it.
I suggest we keep the DCS support we already have, at least for now, while still adding support for the new private mode.
Using a DCS sequence to enable/disable synchronized updates always struck me as odd. A DECSET private mode seems to make way more sense. That way it can be batched with other private mode params and also benefit from XTSAVE and XTRESTORE.
Deleting a branch is permanent. It CANNOT be undone. Continue?