Patches from Tony Peden to separate property XML I/O operations into a

Patches from Tony Peden to separate property XML I/O operations into a
separate header file.  This change will help integrate properties into
JSBSim.

Also, I (David Megginson) removed most of the SimGear include
statements from globals.hxx, reducing the amount of recompilation
every time SimGear changes.  This required making minor changes to a
lot of files that were depending on the side-effects of the inclusions
in globals.hxx.