[Doc] imgui_demo.cpp: Add DEMO_MARKERS and DemoCodeWindow

This commit provides the ability to pinpoint the exact location of
the code of all the demos in imgui_demo.cpp: when the "Code Lookup"
checkbox is ticked, the code location (file and line number)
of any demo currently hovered by the mouse will be shown in a tooltip.

Furthermore a basic code viewer is provided (which reads the code from
the imgui_demo.cpp file); as well as a "View on github" button.

Implementation notes:
- This feature is provided by the "DEMO_MARKER" macro.
  See example usage below
- Most of the code for this feature is grouped inside
  the namespace `DemoMarkerTools`.

Origin:
These modifications are the basis for a more complete interactive
imgui manual (https://pthom.github.io/imgui_manual_online/manual/imgui_manual.html).

This manual was first presented in the "Gallery/screenshots (part 11)"
discussion (ocornut#3075)

// Example usage of the DEMO_MARKER macro (with hierarchical demos):

     DEMO_MARKER("Widgets");                      // For clarity, calls to the DEMO_MARKER macro
     DEMO_MARKER("Widgets/Basic");                // should try to be hierarchical (via the use of "/" separators)
     ...

     DEMO_MARKER("Widgets/Basic/Checkbox");       // Here the call to DEMO_MARKER will start a new rectangular bounding
     static bool check = true;                    // around the next widget(s). Whenever the user clicks the "Help/Code Lookup"
     ImGui::Checkbox("checkbox", &check);         // button and then click this bounding, he will be shown the source code location,
                                                  // of this DEMO_MARKER call.

     DEMO_MARKER("Widgets/Basic/RadioButton");    // End of the previous bounding, start of a new one
     static int e = 0;
     ImGui::RadioButton("radio a", &e, 0); ImGui::SameLine();

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