fix: Hide icon when ShowIcon = false

[RussKie]

Fixes #3022

Proposed changes

• Correct a regression introduced in Cleanup Form.IsRestrictedWindow #695 when a condition was flipped that stopped setting form's extended style flag WS_EX_DLGMODALFRAME.

Customer Impact

Customers now able to hide form's icon at runtime by setting myform.ShowIcon = false.
Workarounds involve either

• setting an icon on a form to a transparent blank icon, or
• performing an interop call to hide the icon before hiding the icon:

  if (myform.ShowIcon)
  {
      int extendedStyle = (int)User32.GetWindowLong(myform.Handle, User32.GWL.EXSTYLE);
      User32.SetWindowLong(myform.Handle, User32.GWL.EXSTYLE, (IntPtr)(extendedStyle | (int)User32.WS_EX.DLGMODALFRAME));
  }
  
  myform.ShowIcon =false;

Regression?

• Yes, introduced in .NET Core 3.0

Risk

• Minimal

Test methodology

• manual tests in the integration test app
• unit tests

Microsoft Reviewers: Open in CodeFlow