[FEATURE] Add command line support plus parameters for emulator launch (adding Steam Overlay support, among other benefits)

[notNSANE]

What problem does it solve?

The biggest problem, besides some specific emulator quirks that aren't supported; is not being able to launch Steam's RetroArch version as a main app and not a child of NeoStation.

Currently all and any frontends launch Steam's RetroArch as a child, causing Steam to hook to the frontend instead of RetroArch. This has two main issues:

1. Having to configure yet another gamepad configuration (as a non-Steam app) instead of using RetroArch's. This one is a major annoyance for Steam Controllers which need to live inside the Steam ecosystem.
2. Not using Steam's RetroArch community features -- all videos, screenshots will be added to a non-Steam app instead of RetroArch's. Also, no Steam Guides or forums support.

Proposed Solution

Easiest way would be to have an option to use our own command line to launch an emulator, could be added to the System Settings UI (or maybe the Game Settings UI too -- perhaps useful for some emulators?). This window:

The idea is to let us customize what happens when the user starts a game. The following are examples on how to add Steam Overlay support to ES-DE and PlayNite.

This is how a custom command line looks in ES-DE:

%RUNINBACKGROUND% %EMULATOR_STEAM% -applaunch 1118310 -L bsnes_libretro.dll %ROM%

And this is how it looks on PlayNite:

-silent -applaunch 1118310 -L ".\cores\bsnes_libretro.dll" "{ImagePath}"

As we can see here, the command -applaunch is forcing Steam to launch RetroArch as a new main app, not as a child of the frontend.

And what's important too, is the custom parameters, in this case being %ROM% and {ImagePath}, which tells RetroArch there the game is located.

Alternatives Considered

Right now the only solution is using ES-DE or PlayNite. Both frontends support command line.

Additional Context

It's important to mention that this issue is a major blocker for Steam Controller users since they're forced to use Steam Overlay for the controller to hook properly to a game.

Some links with more information:

PlayNite: https://www.reddit.com/r/playnite/comments/16qnieq/tutorial_how_to_launch_retroarch_steam_version/
ES-DE Steam guide: https://gitlab.com/es-de/emulationstation-de/-/blob/master/USERGUIDE.md#using-the-steam-release-of-retroarch

[notNSANE]

and before I forget AGAIN, this feature was available on the very first NeoStation builds, but it got removed. I remember it crashed RetroArch for some reason.