Enhancement: Provide option to control tab elements

[eejd]

I think it would be useful to provide the option for what to display in the tab bar and control what happens as tabs increase and size decreases. Specific initial element request is to provide the tmux session name as an option to display. Also would be good to provide the option to reduce from full connection to just user@host as space decreases. May relate to #241.

[kitknox]

In regular sessions we really only have the choice to render information derived from the connection (ie user@host) or the OSC provided metadata. The OSC metadata you can ultimately customize as we just update with whatever you emit there. For tmux control mode we have a bit more of a choice as we have user@host of the gateway, the tmux metadata itself (which is generally updating from OSC info), and then any other OSC escape sequences that are passed on.

There is currently also an option in the context menu where you can rename a tab which overrides any of the above for the duration of that tabs lifetime.