Add API for querying registering/unregistering input processor

[dzeligmanriot]

We use these API in a subsystem (that maybe should exist in the root plugin in some form? ) to unregister and register during breakpoints etc.

Some example code:

void UDebugGUISubsystem::HandleScriptException(const UObject* InObject, const FFrame& InFrame,
	const FBlueprintExceptionInfo& InInfo)
{
	if (InInfo.GetType() == EBlueprintExceptionType::Breakpoint)
	{
		UnregisterInputProcessor();
	}

	if (InInfo.GetType() == EBlueprintExceptionType::AccessViolation
		|| InInfo.GetType() == EBlueprintExceptionType::AbortExecution
		|| InInfo.GetType() == EBlueprintExceptionType::FatalError )
	{
		DisableGUI();
	}
}

`void UDebugGUISubsystem::HandlePausePIE(bool bIsSimulating)
{
	UnregisterInputProcessor();
}


void UDebugGUISubsystem::HandleResumePIE(bool bIsSimulating)
{
	RegisterInputProcessor();
}`

void UDebugGUISubsystem::UnregisterInputProcessor()
{
	if(bDebugGuiOpen && ImGui::GetCurrentContext() != nullptr)
	{
		TSharedPtr<FImGuiContext> ActiveContext = FImGuiContext::Get(ImGui::GetCurrentContext());
		if(ActiveContext.IsValid())
		{
			ActiveContext->UnRegisterInputProcessor();
		}
	}
}

void UDebugGUISubsystem::RegisterInputProcessor()
{
	if(bDebugGuiOpen && ImGui::GetCurrentContext() != nullptr)
	{
		TSharedPtr<FImGuiContext> ActiveContext = FImGuiContext::Get(ImGui::GetCurrentContext());
		if(ActiveContext.IsValid())
		{
			ActiveContext->RegisterInputProcessor();
		}
	}
}