Terminal: Add support for tab tearoff and tab merge

[DHowett-MSFT]

I'm repurposing this for tab reordering and demoting it to a Task. We'll track the (much larger) workitem to do tab tearoff as a deliverable for vBacklog.
Notes/reasoning:

• Tab reordering is easy.
• Tab tearoff requires us to answer the following questions:
  
  
  
  • WT.exe is a multi-instance app. How do we drag tabs between _different processes?_
  
  
  • Should WT be multi-instance? We may want to single-instance it.
    
    
    
    
    
    • If we want to single-instance it we need to design "new window" (part of #1051)
    
    
    
  
  
  • We still need to design "new window" for tearoff-without-merge because we need a drop target for the old tab.
  
  

_Originally posted by @DHowett-MSFT in https://github.com/microsoft/terminal/issues/443#issuecomment-501928756_

[paradonym]

also to have a sort of way to manage many open tabs... If I have 3 WSL tabs 1 cmd and two powershells tabbed it get's out of the UI at the right side of the tab bar (so I can't close the last tabs unless I close the first ones).

[cdmihai]

This would be so useful. I constantly open new tabs on some focused work, start splitting panes within the tab, and then at some point decide to move the whole tab and panes (right now I just recreate the panes and their directories from scratch) to a new terminal window in a new virtual desktop.

[adgustaf]

Hi,

Any progress on this? Just wanted to say I find this to be very useful.

Thanks for all your hard work on this product!

[DHowett-MSFT]

Thanks for asking! Unfortunately, there hasn’t been any progress. When there is, this issue will be the first to be updated about it.

[adgustaf]

Thanks for the info! Okay I'll be following this issue :)

[pcgeek86]

I just tried to tear a tab off a window in the Windows Terminal Preview version and was unable to. That led me to searching for this issue.

Here's my use case: I am trying to demonstrate connecting two separate tmux clients to the same session, and running at different window sizes. In order to effectively do this, I need to resize one tmux window to separate dimensions from the other one. Since both my Ubuntu WSL tabs were running in the same Windows Terminal window, I was unable to do this. Hence, I tried to "tear" one of the existing tabs off into its own Windows 10 window.

This is where I got stuck, and had to open a brand new Terminal window instead, and then open up an Ubuntu tab in that window.

The work-around isn't hard, but it's just convenient to be able to tear off existing tabs, just like you can in Google Chrome.