GIG Player now uses correct velocity and changes sample rate properly

Fixed crashes when loading invalid file, but this required exceptions
since libgig throws exceptions, hence the changes to CMakeLists.txt.

If the sample rate is changed (e.g. on exporting at 48k when the samples
are at 44.1k), when playing the note convert each note individually to
the correct sample rate instead of converting the output from the notes
to the correct sample rate.

Save the instrument so we don't have to search the entire GIG file every
time we play a note. This should make it work better on computers that
don't have tons of RAM.

Currently there is a 0.5 second fade out time when the note is released.
I still have to implement the release-triggered samples.