Use of "gamelist-only" should disable "file exists" checks on startup

[Enverex]

Copying this over from the original here - Aloshi#401

I took it for granted that this was how it worked by default but apparently not.

When using "gamelist-only", ES should not bother scanning for files itself, it should assume that the gamelist is correct (hence telling it just to use the gamelist and not scan). The rom folder and file extension options in the es_systems file shouldn't matter when using this flag as they won't be used.

This would also solve issue Aloshi#370 as it wouldn't matter what the ROM file path was set to, it would just "run" it without checking.

Not 100% sure if this still applies to your new branch, but I didn't see anything addressing it directly when I scanned through.

[Enverex]

I'd actually like to add that this renders EmulationStation almost unusable when using a large collection because it takes literally minutes to open in that case.

[MCMic]

Hello,

I’m trying to use ES to launch native games, by using a script which turns desktop menu entries into a gamelist.xml file.
And I have the same problem, the file check is run on the path so the games won’t appear in the list.
It would be useful if --gamelist-only could bypass file check, as well as path and extension options from es_systems (I had to put dummy values in those)

[kuangmk11]

We should be able to set this per system in es_systems.cfg. Having a tailored gamelist is great/necessary for MAME and others that use their own file name convention but a hassle for more basic console systems where a descriptive filename itself will suffice.