fix(lutris): game and game box art detection

[Rolv-Apneseth]

Following issues fixed:

1. games that don't have a full path specified for their exe field were being skipped since the executable path could not be parsed
2. box art that used a format other than jpg, or that used the slug field instead of the game_slug field for their name, would not be found

For now, png, jpg and jpeg are the only formats that will be checked but it will be trivial to add support for other formats if it is requested.

Edit: fixes #7