Factor out winesteam prefix creation
Add comment about the prefix subdirectory in winesteam
create_prefix() can figure out the parent itself
Simplify prefix handling code
Add missing prefix creation in winesteam install
That method name was lying about its intentions
Now we can have a method with no side effects
Default to winesteam's default prefix in is_installed
Merge branch 'next' of https://github.com/lutris/lutris into next

Conflicts:
	lutris/runners/winesteam.py