Make it possible to load/save input mappings.

Settings these up can be a bit of a hassle if you have a lot of them,
so we allow them to be saved to disk and then loaded back (complete
with machinery for fuzzy-matching devices if the state has changed,
e.g. cards moved around).

This introduces a dependency on protocol buffers, because it's a
convenient way to store simple data to disk (including forward
compatibility). We choose to use the text format for the user's
convenience; we don't need the speed or compactness of the binary format.