globals.cxx -- FGGlobals::set_fg_scenery():

globals.cxx -- FGGlobals::set_fg_scenery():
    Insert empty string as marker between FG_SCENERY path elements.
    FG_SCENERY=A:B expands to [A/Terrain, A/Objects, "", B/Terrain, B/Objects, ""]
    (assuming that both A/ and B/ have Terrain/ and Objects/ subdirs).

tileentry.cxx -- FGTileEntry::load():
    Check all tile dirs in FG_SCENERY from left to right: add all objects
    to the scenery until a terrain tile was found: In this case read the
    rest of that group (i.e. the Objects/ twin dir) and then stop scanning.

    Better structuring of log messages & fix warnings.