Support leader-followers broadcast

[daonb]

What's missing? Please describe your flow and where this feature will help.
We need a clear protocol for when more than one clients is connected to the same server.

In the proposed leader-followers scheme a session can have no more than one leader
and as many followers as the server can handle. The followers scale their screens
based on the dimensions in the payload they get from the server.
Followers can't zoom in, split panes, move borders, or delete panes.
The relevent buttons are greyed out and if clicked, a popup shows:

"Terminal7 is in follower mode"

With two buttons: Keep Following & Lead

Followers don't show the dividers.

Additional context
Gate.fitScreen needs to be refactored - renamed to leader and it's value changed only when
the user clicks the greyed buttons, or the Reset button and selectig "Become a leader"