to find out whether there has ever been a snapshot taken with a given
configuration version.
Implemented an "on snapshot" flag, which indicates, whether the current
state equals to some snapshot. If so, a new snapshot is not taken
in upgrade / downgrade case.
compatibility check:
A release Slic3r is not compatible with alpha and beta configs,
a beta Slic3r is not compatible with alpha configs, but is compatible
with beta configs etc.
Added Version Index "version" method.
Implemented automatic selection of default_print_profile and
default_filament_profile, when the print / filament profiles are
not compatible with the selected printer profile.
Fixed selection of a printer profile, if the currently selected
printer profile becomes invisible.
