[UI] Add a logger

[liubai01]

This is a feature request for a basic logger.

Background

• The framework of the code is in CoreLoggerWidget.cpp
• The GUI of the editor is based on Dear ImGui.
• Logger bases on spdlog.
• Logger helps users debug. The output of other components/widgets should be in the logger.

Goal

• Migrate the logging example in imgui_demo.cpp to CoreLoggerWidget.cpp.
• Use spdlog as backend.

References and guidance

Show the widget

To show the CoreLoggerWidget.cpp widget, toggle Widget/Logger in menubar.

Quick-start

For example, add a line of ImGui::Text("Hello") in CoreLoggerWidget.cpp \ void CoreLoggerWidget::Update().

You could then see a line of text in that.

Use user-defined sink spdlog as backend

There is a simple example of using spdlog in Debugout.h.

ImGui Front-end example

Dear ImGUI demo shows an example of a log(imgui_demo.cpp/ShowExampleAppLog(...)).

You could uncomment ImGui::ShowDemoWindow(); to show example of widgets of dearImGui in CoreApp.cpp.

void CoreApp::BeforeUpdate()
{
    mGUIManager->Update();
    //ImGui::ShowDemoWindow();
}

A deeper introduction to constructing a widget could refer to this video (in Mandarin Chinese).