Push packaging down to shoes-swt

Packaging was previously baked into the `Shoes::CLI` class, which lived up
in `shoes` (and now in `shoes-core`). We don't want JRuby-specific dependencies
at that level, though, so this belonged down in `shoes-swt`.

We accomplish this by creating a DSL and backend class to represent
packaging, and call those from the CLI. This lets us move the `furoshiki`
dependency down to `shoes-swt` as well... extra win!
Yeah, on further reading, it would be clearer to have those explicitly defined on the DSL class and make the delegation more obvious. Also, agreed that the parameter data itself could be DSL level, not SWT here.

On `run?`, would you prefer `should_package?`