Add imgui demo to the debug menu

[moxian]

Summary

Infrastructure "Added Imgui demo screen (and style editor) to the debug menu"

Purpose of change

Breaking the UI without the need to recompile and/or painstalkingly step line-by-line with a debugger trying to figure out why is my text clipping ever so slightly.

Describe the solution

Shamelessly copy-paste the imgui demo code on the main menu screen into the debug_menu.cpp (without the lorem ipsum window from the former)

Describe alternatives you've considered

Adding it to the global hotkeys so that I can invoke it while having another imgui menu open, but I'm hesitant to pollute global hotkeys with functionality that's exclusively useful for debug purposes. (I'm also not 100% sure it will even re-draw the "other" imgui windows that way, so the point is moot. But i haven't tried either)

Testing

Yes, that's kinda the point.

Additional context

[mqrause]

Instead of duplicating it, you should put the class in a header.

[moxian]

Reasonable. But which one? Neither including debug_menu.h in main_menu.cpp nor including main_menu.h in debug_menu.cpp makes a lot of sense.
Would it be okay to create a new imgui_demo.h + .cpp (or perhaps debug_imgui_demo) just for this (and include that in both debug_menu.cpp and main_menu.cpp)?

[mqrause]

A new file is fine, yeah.

[Brambor]

It's preferable to keep the name imgui_demo over changing it to debug_imgui_demo.

(Just making sure the name change isn't merged, maybe you knew already.)