Refactoring get_game_path and stuff (v2)

I reworked and finished this, normally it's all good.

So the goal was to use get_game_path() as the game's directory and only this.
After these changes:
* get_game_path() becomes game_path (property).
* It returns "directory" from the pga database by default. It does it by querying the db. The thing is, this query is done in Game too, so perhaps it's not efficient to read twice the game's db entry for each game at Lutris launch ? Then we would need to pass game_data from Game to Runner alongside config.
* Browse Files is fixed for all runners.
* working_dir has been introduced, specifies the working directory when launching games.
* Fixed game launch not resuming after runner install.