This allows to have generic axis/button definitions with aircraft
specific or aircraft class specific modification overlays. These
are also found in $FG_HOME, where they override global files. This
mechanism is desirable, because the Warthog is an A-10 replica, and
it should be possible to have an automatically loaded (1:1 mapped)
A-10 joystick config when flying the A-10, while using generic layouts
otherwise, or a specific helicopter overlay for helicopters etc.
Overlay files look exactly like joystick config files, except they
have no <name> and only specify actually differing elements. Axes
with <number> *need* a property index (e.g. <axis n="2">), which
usually corresponds to the <unix> value (i.e. n'th axis def in file).
Overlay files can contain a <nasal> block which is executed on
load. The files are to be named {$FG_ROOT,$FG_HOME}/Input/Joysticks/\
ThrustMaster/Warthog/{Joystick,Throttle}/{generic,helicopter,<aircraft>}\
.xml.
