Correct `TextBox.PlaceHolderText` implementations

Is your feature request related to a problem? Please describe.

The current implementation of TextBox.TextPlaceHolderText API added in #96/#238 suffers from a number of deficiencies and shortcomings as listed below.

1. It is unaware of the native implementation and visually inconsistent with it:

   textBox1.PlaceholderText = "Custom!";
   SendMessageW(textBox1.Handle, EM_SETCUEBANNER, IntPtr.Zero, "NATIVE PlaceHolder..");
   

   

2. It is visually inconsistent with the native implementation:

   • This implementation - TextBox control with PlaceHolderText property set:

   • Native implementation - TextBox control with the placeholder text set via the EM_SETCUEBANNER:

     • different locations,
     • different font colours, and
     • different shortening strategies.

3. The current implementation is also not UIA aware:

   • This implementation
   • Native implementation:

Describe the solution you'd like and alternatives you've considered

• Keep the existing public API surface in tact
• Rework PlaceHolderText implementation as follows:
  • Use Win32 API EM_SETCUEBANNER for single line textboxes when visual styles are enabled.
  • Align location of the custom drawn text with the position of the Win32 implementation.
  • Align the font of the custom drawn text with the size/style of the Win32 implementation.
  • Align the color of the custom drawn text with the color of the Win32 implementation.
  • Align the shortening strategy of the custom drawn text with the color of the Win32 implementation - i.e. clip the text instead of "..."
• Fallback to the current implementation in the following situations:
  • TextBox.Multiline = true.
  • Visual styles are disabled.

❗ Tests must verify correct behaviours when switching single<-->multiline.

❕ Accessibility support for the custom drawn text at this stage remains optional.