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 Feature Request: Key binding for "New Window" #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 #443 (comment)

[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.