Rework TermControl's initialization (#5051)

This commit rewrites a large swath of TermControl's initialization code.

* `TermControl` now _always_ has a `_terminal`; it will never be null
* Event registration for `_terminal` and any other available-at-init
  fixtures has been moved into the constructor.
* Event handlers how more uniformly check `_closing` if they interact
  with the _terminal.
* Swap chain attachment has been cleaned up and no longer uses a
  coroutine when it's spawned from the UI thread.
   * We have to register the renderer's swapchain change notification
     handler after we set the swap chain, otherwise it'll call us back
     when it initializes itself.
* `InitializeTerminal` now happens under the `_terminal`'s write lock
   * Certain things that InitializeTerminal were calling themselves
     attempted to take the lock. They no longer do so.
* TermControlAutomationPeer cannot take the read lock, because setting
  the scrollbar's `Maximum` during `InitializeTerminal` will trigger
  vivification of the automation peer tree; if it attempts to take the
  lock it will deadlock during initialization.
* `BlinkCursor` was renamed to `CursorTimerTick` because it's the "Tick"
  handler for the "CursorTimer".
* `DragDropHandler` was converted into a coroutine instead of just
  _calling_ a coroutine.

Caveats:

Terminal may not have a `_buffer` until InitializeTerminal happens.
There's a nasty coupling between RenderTarget and TextBuffer that means
that we need to have a renderer before we have a buffer.

There's a second nasty coupling between RenderThread and Renderer: we
can't create a RenderThread during construction because it needs to be
given a renderer, and we can't create a Renderer during construction
because it needs a RenderThread. We don't want to kick off a thread
during construction.

Testing:

I wailed on this by opening and closing and resizing terminals and panes
and tabs, up to a hundred open tabs and one tab with 51 panes. I set one
tab to update the title as fast as it possibly could and tested
teardown, zoom, resize, mouse movement, etc. while this was all
happening.

Closes #4613.