Add partial support for custom mouse cursors

[Sixze]

This PR adds support for custom mouse cursors, but only for InGui windows.

To implement custom mouse cursors inside the game viewport, a custom UGameViewportClient with overridden GetCursor() and IsInPermanentCapture() functions is required. Here is an unedited code from my project that can be used as an example:

EMouseCursor::Type ULmGameViewportClient::GetCursor(FViewport* InViewport, const int32 MouseX, const int32 MouseY)
{
#ifndef IMGUI_DISABLE
	const auto MouseCursor{Super::GetCursor(InViewport, MouseX, MouseY)};
	if (MouseCursor != EMouseCursor::None && MouseCursor != EMouseCursor::Default)
	{
		return MouseCursor;
	}

	const auto* DebuggerSubsystem{ULmDebuggerSubsystem::Get(GetWorld())};
	if (!IsValid(DebuggerSubsystem) || !DebuggerSubsystem->IsDebuggerEnabled())
	{
		return MouseCursor;
	}

	if (DebuggerSubsystem->IsGameInputAllowed() && MouseCursor == EMouseCursor::None)
	{
		// Preserve cursor visibility if input is not enabled.
		return MouseCursor;
	}

	const auto& WorldContext{GEngine->GetWorldContextFromGameViewportChecked(this)};
	const ImGui::FScopedContext ImGuiContext{WorldContext.PIEInstance};

	if (!ImGuiContext)
	{
		return MouseCursor;
	}

	return ImGui::ConvertMouseCursor(ImGuiContext->GetLastMouseCursor());
#else
	return Super::GetCursor(InViewport, MouseX, MouseY);
#endif
}

bool ULmGameViewportClient::IsInPermanentCapture()
{
#ifndef IMGUI_DISABLE
	const auto* DebuggerSubsystem{ULmDebuggerSubsystem::Get(GetWorld())};
	if (IsValid(DebuggerSubsystem) && !DebuggerSubsystem->IsGameInputAllowed())
	{
		return false;
	}
#endif

	return Super::IsInPermanentCapture();
}