teach Qt app to launch stuff relative to current directory:

* avoids issues with spaces in path to working directory
ignore os x native stuff
Fix FX synth naming:

don't resolve the name of a synth until trigger_synth
Add new functions: use/with_sample_pack_as for aliasing samples:

You can now assign prefix aliases for samples. Consider:

use_sample_pack_as "/foo/bar/baz", :bazzers
use_sample_pack_as "/foo/bar/boz", :boozers

sample :bazzers_eggs    #=> plays "/foo/bar/baz/eggs.wav"
sample :bozzers_cheese  #=> plays "/foo/bar/boz/cheese.wav"

with-* equivalent works for the specified do/end block.

Thanks to Daniel Wagner for the suggestion.