Add an ability to control Windows rounded corners behavior

[BAndysc]

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

On Windows, rounded corners are currently requested at this location:

Avalonia/src/Windows/Avalonia.Win32/WindowImpl.cs

Lines 1157 to 1158 in 58de42b

	int cornerPreference = (int)DwmWindowCornerPreference.DWMWCP_ROUND;
	DwmSetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_WINDOW_CORNER_PREFERENCE, &cornerPreference, sizeof(int));

However, to maintain the aesthetic of the Classic theme, DWMWCP_DONOTROUND should be applied instead. I would still like to use SystemDecorations=Full to retain TitleBar dragging and window resizing functionality.

Describe the solution you'd like

Introducing a new StyledProperty PreferRoundedCorners on the Window could handle this, allowing flexibility within this method.

Describe alternatives you've considered

While I can manually call DwmSetWindowAttribute in the ExtendClientAreaToDecorationsChanged event, having a dedicated property would make it simpler.

Additional context

No response

[thevortexcloud]

PreferRoundedCorners

It should probably follow the current hint pattern to indicate it may not work/do anything.