Create working_dir base
working_dir for Linux runner
working_dir for Dosbox
Refactor: Make use of new game_exe property
Remove self.game_path remains
Remove some leftover get_game_path
Remove unused stuff in Runner
Fix Linux.play() using relative path for the executable
Add working_dir setting to Wine and Linux runners
Docstrings cleanup
Return game directory in get_game_path()

Very simple actually, just needed to read the pga db from Runner.
Though 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.
Check if runner is installed before browsing files

Runners concerned: steam, winesteam, desura.

Also makes game launch resume (with launching the game) after the
runner has been successfully installed.
Rename get_game_path() => game_path