launcher: Split out launcher implementations into three distinct ones

We now have a launcher interface and distinct implementations for
logind, weston-launch, and direct DRM, each in their own files.

This helps up clean up the spaghetti code into something that's
hopefully a bit more understood. There should be no functional
changes here.

Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

update:  Dropped redundant free() in weston_launcher_destroy()