Avoid focus loops in ConPTY (#17829)

This fixes a lot of subtle issues:
* Avoid emitting another de-/iconify VT sequence when
  we encounter a (de)iconify VT sequence during parsing.
* Avoid emitting a de-/iconify VT sequence when
  a focus event is received on the signal pipe.
* Avoid emitting such sequences on startup.
* Avoid emitting multiple such sequences
  when rapidly un-/focusing the window.

It's also a minor cleanup, because the `GA_ROOTOWNER` is not security
relevant. It was added because there was concern that someone can just
spawn a ConPTY session, tell it that it's focused, and spawn a child
which is now focused. But why would someone do that, when the console
IOCTLs to do so are not just publicly available but also documented?

I also disabled the IME window.

## Validation Steps Performed
* First:
  ```cpp
  int main() {
      for (bool show = false;; show = !show) {
          printf(show ? "Show in 3s...\n" : "Hide in 3s...\n");
          Sleep(3000);
          ShowWindow(GetConsoleWindow(), show ? SW_SHOW : SW_HIDE);
      }
  }
  ```
* PowerShell 5's `Get-Credential` gains focus ✅
* `sleep 5; Get-Credential` and focus another app. WT should start
  blinking in the taskbar. Restore it. The popup has focus ✅
* Run `:hardcopy` in vim: Window is shown centered at (0,0) ✖️
  But that's okay because it does that already anyway ✅
* `Connect-AzAccount` doesn't crash PowerShell ✅

.github/actions/spelling/allow/apis.txt

@@ -96,6 +96,7 @@ IGraphics
 IImage
 IInheritable
 IMap
+imm
 IMonarch
 IObject
 iosfwd