Add an additional visibility flag to the menubar implementations,
conditional on whether or not the menubar overlaps the window content.
(I.e for PUI but not Cocoa). This flag is linked to a new property
/sim/menubar/overlap-hide, which the renderer drives off the splash-
screen visibility.
Restarting the GUI could cause Cocoa menus to misbehave, due to
destruction behaviour of SGBinding. Use new clear() helper in the
short term to work around this.
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.
