Backends: amends to 1db1066 + merge minor bits from docking incl SetActiveIdUsingNavAndKeys().

No need to clear fields before deletion. DX12: renamed to match docking branch.

imgui.cpp

@@ -7170,6 +7170,7 @@ void ImGui::PopTextWrapPos()
     window->DC.TextWrapPosStack.pop_back();
 }
 
+// FIXME: We are exposing the docking hierarchy to end-user here (via IsWindowHovered, IsWindowFocused) which is unusual.
 bool ImGui::IsWindowChildOf(ImGuiWindow* window, ImGuiWindow* potential_parent)
 {
     if (window->RootWindowDockTree == potential_parent)