Allow Splitting dockId multiple times

[MegaMech]

This allows splitting a dock node so that you can create a left/right/up/down docking location programmatically.

Without this, the programmer can only split one direction off of the main dockId which prevents docking multiple windows to the other sides of the main program window.

Example:

    const ImGuiID dockId = ImGui::GetID("main_dock");
    if (!ImGui::DockBuilderGetNode(dockId)) {
        ImGui::DockBuilderRemoveNode(dockId);
        ImGui::DockBuilderAddNode(dockId, ImGuiDockNodeFlags_NoTabBar);

        ImGui::DockBuilderDockWindow("Main Game", dockId);

        ImGuiID rightId = ImGui::DockBuilderSplitNode(dockId, ImGuiDir_Right, 0.25f, nullptr, nullptr);
        ImGuiID topId = ImGui::DockBuilderSplitNode(dockId, ImGuiDir_Up, 0.25f, nullptr, nullptr);
        ImGuiID bottomId = ImGui::DockBuilderSplitNode(dockId, ImGuiDir_Down, 0.25f, nullptr, nullptr);
        ImGui::DockBuilderSetNodeSize(rightId, ImVec2(viewport->Size.x * 0.2f, viewport->Size.y));
        ImGui::DockBuilderSetNodeSize(topId, ImVec2(viewport->Size.x, viewport->Size.y * 0.1f));
        ImGui::DockBuilderSetNodeSize(bottomId, ImVec2(viewport->Size.x, viewport->Size.y * 0.1f));
        
        ImGui::DockBuilderDockWindow("Scene Explorer", rightId);
        ImGui::DockBuilderDockWindow("Tools", topId);
        ImGui::DockBuilderDockWindow("Content Browser", bottomId);

        ImGui::DockBuilderFinish(dockId);
    }

    ImGui::DockSpace(dockId, ImVec2(0.0f, 0.0f), ImGuiDockNodeFlags_None | ImGuiDockNodeFlags_NoDockingInCentralNode);