Version keybindings file.

Version keybindings file.

Allow to run both Ardour 5.5 and later (changed bindings name)
with the same config folder.

Like with .color files, custom .keys files won't be taken into account
when updating to a new version.

This is mostly a stopgap solution. Eventually we'll have to come up with a
merge&prune mechanism for bindings or maintain a separate version for
these files. Hopefully some better idea will come up...