Allow developers to create custom dev mode menus

It would be really nice to get the ability to add custom windows to the dev
mode menu. It looks like there are already things in place that would allow
for this. The aircraft editor window is defined in an XML file located in
Official\Steam\fs-devmode\Edition\FileDefs. Having the ability to do this
would be extremely useful and would allow developers to add things that would
be helpful in their development. If it’s possible, it would also be great to
define custom behavior for these menus with ImGui bindings in WASM (not sure
if ImGui allows for something like this)