$FG_INSTALL_PREFIX represents the FlightGear installation prefix, such
as /usr, /usr/local or /opt/FlightGear on Unix systems. Copying the
--help output and translated strings there avoids having to write to
$FG_ROOT when 'make install' (or some OS-dependent equivalent) is run
from the FlightGear build directory---that would be ugly when $FG_ROOT
points to the FGData Git repository.
In FGLocale::FGLocale(), Translations/locale.xml is loaded using
readProperties() and fatalMessageBox() (in case an error is
encountered). Note that it couldn't be loaded via fgLoadProps() in the
current state, because this function relies on guiErrorMessage() when an
error is encountered, which calls mkDialog(), which itself does
globals->get_subsystem("gui"). This last call can't be done from
FGGlobals' constructor---where the 'globals' pointer is still
NULL---hence the need for a different mechanism not relying on
FGGlobals.
For consistency, and also because it provides a better user experience[1],
load options.xml using the same method instead of with fgLoadProps().
[1] I.e., in case of an error, the user gets to see a graphical popup
window with an explanatory message before FG exits, assuming he is
either on Windows, or on Mac, or has Qt support built in FG, as
opposed to only an SG_LOG() call [because when options.xml is
loaded, guiErrorMessage() used by fgLoadProps() can't use the 'gui'
subsystem].
Remove uses of .str(), .c_str() and some other methods of SGPath.
Pass SGPath directly where possible, or explicitly convert to the
appropriate 8-bit encoding.
Respect the user's language selection in the system settings, and try those languages in turn. If a language is explicitly specified, try that first regardless.
