Bugfix: Non-existent AIModels could trigger a request to load scenery tiles

Bugfix: Non-existent AIModels could trigger a request to load scenery tiles
at (lon, lat) coordinates -1000,0. This patch fixes the AIModels/Traffic
Manager side of things. The AIModels subsystems allowed the creation of
AIAircraft with non-existent 3D models. If such a model didn't exist, the
aip class didn't get initialized, resulting in the above-mentioned bogus
position information. Here I circumvent this problem by a) only interacting
with the tile loader if the model is visible (and hence has succesfully been
initialized) and b) by disallowing the traffic manager to create AIAircraft
objects if the path to the 3D model doesn't exist.