Add Atari 2600 5200 7800 and Lynx

[Paradox]

Atari2600 was, long ago, merged with the ATARI7800 core, and, the rom files need to be in the ATARI7800 folder for mgl to work, until something is changed in the cores/mgl code, you can use the pathsextra fields to separate them into separate directories, under ATARI7800 if you wish, which will keep the gamelist creator from scanning both sets of files for each core.

usedefaultpaths only takes a couple of seconds, so, is not "slow" and will be enabled by default. You can keep it set to No or, set it to No if you'd like, and, it will continue to work, somewhat, but, will become unsupported.

Everdrive mappings have been made obsolete, and have been removed, this affects nobody, as we don't use them for anything.

There is still work to be done, for example, many Atari 5200 games need a left or right dpad press to start the game, and this functionality will take a great deal of research to figure out how to approach this, or, how to exclude them from the gamelists, but, the vast majority of them seem to work fine, as is

[Paradox]

This has been obsoleted by #153 but, if you do merge this one, first, the other one should still merge, without errors.