[Feature request] Look for .srm files in addition to .sav files when loading a ROM

Relates to https://github.com/libretro/mgba/issues/296 - syncing retroarch with non-retroarch emulation is difficult simply because retroarch dumps SRAM using the .srm extension rather than the .sav extension.

It should be fairly easy to allow either loading arbitrary filetypes as saves or just adding .srm files to the one-element save filetype filter list. It is less trivial to write code for deciding what extension to save a new memory dump with, or to decide what to use as the actual source if both a .sav and .srm file are present, but simply adding the ability to optionally read .srm files while leaving everything else the same would still be very welcome.