Fix the include context when -set.xml are parsed by the launcher to
extract states and other data. Ensure aircraft paths are available
and handled consistently with normal -set.xml parsing. Without this,
many -set.xmls fail to parse.
Sentry-Id: FLIGHTGEAR-4H
Sentry-Id: FLIGHTGEAR-3B
Sentry-Id: FLIGHTGEAR-3R
Sentry-Id: FLIGHTGEAR-3Q
Sentry-Id: FLIGHTGEAR-3F
Sentry-Id: FLIGHTGEAR-2R
Sentry-Id: FLIGHTGEAR-65
Sentry-Id: FLIGHTGEAR-42
Automatic selection in the launcher is disabled for now, since
it needs more testing before release, but the basic UI for selection
is straightforward enough to throw in.