[Feature Request]Auto select rom if present in appimage path

I would love for the rom to be automatically selected if the rom file is located in the same path as the executed appimage on linux. I'm attempting to run on Batocera and I'm unable to supply a path as the rom selection window never displays. Conversely, with SoH and 2S2H, it automatically locates the rom and does its magic.

Start by tracing the Linux AppImage startup path and the existing ROM-selection flow. Verify how the executed AppImage path is exposed and test on Batocera; done means a ROM beside the AppImage is selected automatically when the ROM selection window cannot be shown.

Written by the indexing model from the issue text.