## Summary of the Pull Request
Add support for "focus" mode, which only displays the actual terminal content, no tabs or titlebar. The edges of the window are draggable to resize, but the window can't be moved in borderless mode.
The window looks _slightly_ different bewteen different values for `showTabsInTitlebar`, because switching between the `NonClientIslandWindow` and the `IslandWindow` is _hard_.
`showTabsInTitlebar` | Preview
-- | --
`true` | 
`false` | 
## PR Checklist
* [x] Closes#2238
* [x] I work here
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
## Detailed Description of the Pull Request / Additional comments
* **KNOWN ISSUE**: Upon resizing the NCIW, the top frame margin disappears, making that border disappear entirely. 6356aaf has a bunch of WIP work for me trying to fix that, but I couldn't get it quite right.
## Validation Steps Performed
* Toggled between focus and fullscreen a _bunch_ in both modes.
1c8e83d52d
