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.
This allows us to display a platform-native dialog for problems
which occur early in startup (before we can show a PUI/Canvas dialog).
In particular this improves feedback where FG_HOME, FG_DATA or
aircraft selection is wrong, all of which happen very early in startup.
This adds the framework for platform cursor implementations; Windows and X11 to be done.
It also extend the mouse-input code to generate hover events suitable for driving tooltips.
Note there should be no visible functionality change from this commit, since everything
is inactive until fgdata changes are made.
