Add the option to open the ImGui demo from the main menu for debugging

[ZeroInternalReflection]

Summary

None

Purpose of change

The new ImGui windows are a bit tricky to debug and modify because, until contributors (me, I'm talking about me) are familiar with the system, we need to go through a bit of a mental checklist:

• Is this a bug in my code?
• Is this a bug in cata_imgui?
• Is this a bug in ImGui?
• Is this how ImGui is supposed to display things?
• Am I using the wrong ImGui feature for this UI element?
• Can I even do that in ImGui?

Describe the solution

One way to answer several of these questions is with quick access to the ImGui demo screen:

• If it works in the demo screen, then the bug is probably on our end.
• We get to see a bunch of different ImGui elements, and can refer to src/third-party/imgui/imgui_demo.cpp to see a specific example of how they're implemented.
• There's an example of a minimum-viable cataimgui::window implementation that puts things on the screen

Describe alternatives you've considered

• Putting the demo screen under the debug menu
  Then you need to load into the game to see it. The main menu is much faster
• Hiding it behind another option so that only people working on the UI see it
  Possible, perhaps even beneficial, but I haven't done it yet.

Testing

Does the demo screen work perfectly? No, no it does not.

• I needed to tweak the drawing code to give us a way to force things to the back, because otherwise it would be hidden behind a blank ui_adaptor window
• Font loading for CJK characters is definitely broken (see Imgui y/n query popup fails to display translated string on Windows #72162), as the example display widget doesn't work
  
• Curses is a bit twitchy, where the demo screen doesn't show up until after you resize the window
  • Also, the curses demo screen is gigantic and way off to the bottom right, suggesting that our window positioning/resizing isn't working
    
  • There seems to be an odd quirk where the window won't detect a mouse click unless you wiggle the mouse slightly

But this is valuable information for debugging our own windows

Additional context

[katemonster33]

Nice idea! Second thought, since you brought up debugging - add a second option to open the Metrics window (ImGui::ShowMetricsWindow()) this window has lots of helpful state data about ImGui and is very nice when debugging.

[ZeroInternalReflection]

Nice idea! Second thought, since you brought up debugging - add a second option to open the Metrics window (ImGui::ShowMetricsWindow()) this window has lots of helpful state data about ImGui and is very nice when debugging.

I believe that one's accessible from the demo screen under "Tools" (currently on a different branch so I can't check for sure)

Similarly, I saved myself a bunch of grief once I realized having the "Inputs & Focus" widget open would show whatever input ImGui was seeing.