Indicate which pane is focused with the Accent color on the pan… (#3060)

## Summary of the Pull Request

Adds a small border with the accent color to indicate a pane is focused

<img src="https://user-images.githubusercontent.com/18356694/66218711-560e4b80-e68f-11e9-85b0-1f387d35bb92.png" width="480">
<img src="https://user-images.githubusercontent.com/18356694/66218757-6f16fc80-e68f-11e9-8d39-db9ab748c4de.png" width="480">
<img src="https://user-images.githubusercontent.com/18356694/66219194-55c28000-e690-11e9-9835-8b5212e70e8a.png" width="480">

## PR Checklist
* [x] Closes #994
* [x] I work here
* [😢] Tests added/passed
* [n/a] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

I've removed the simple Grid we were using as the pane separator, and replaced it with a Border that might appear on any side of a pane.

When we add a split, we'll create each child with one of the `Border` flags set (each child with one of a pair of flags). E.g. creating a horizontal split creates one child with the `Top` border, and another with the `Bottom`. 

Then, if one of those panes is split, it will pass it's border flag to is new children, with the additional flag set. So adding another Vertical split to the above scenario would create a set of panes with either (`Top|Left`, `Top|Right`) or (`Bottom|Left`, `Bottom|Right`) borders set, depending on which pane was split.

<hr>

* start work on this by tracking border state

* Colorize the border

* Use the accent color for highlighting

* Cleanup the accent color code

* Don't buy any rural real estate when closing a pane

* Closing panes works well now too

* Cleanup for review

* Update src/cascadia/TerminalApp/Pane.cpp

* try some things that don't work to fix the resizing crash

* Revert "try some things that don't work to fix the resizing crash"

This reverts commit 3fc14da.

* this _does_ work, but I think it's not semantically correct

* This doesn't seem to work either.

  I tried adding the pane seperators to the Pane::_GetMinWidth calculation. That
  works for prevent the crash, but the resizing is wonky now. If you add a
  Vertical split, then a second, then resize the middle pane really small,
  you'll see that the _last_ resize doesn't work properly. The text seems to
  overhand into the border.

  Additionally, there's really weird behavior resizing panes to be small. They
  don't always seem to be resizable to the smallest size.

* Revert "This doesn't seem to work either."

This reverts commit 2fd8323.

* Merge the changes from the "this is the one" branch

  Again, no idea what I really did that worked, but it does

* Cleanup from my mess of a commit

  This makes so much more sense now

* Other PR feedback from @carlos-zamora

* Fix a typo