Sound library first iteration

[paulkaplan]

@rschamp I pulled out the UI button that used to be in this PR, so now it implements only the sound library redux actions and component. I documented in @todos all the places this has some strange interactions with the VM. I worked with @ericrosenbaum on the audio loading stuff, when that lands it'll clear up some of those.

Here are the main things I learned going through this

• Sound Library will need to load and play its own sounds. As @ericrosenbaum pointed out, we wouldn't want to reuse the vm.runtime.audioEngine because any effects that are applied would also be applied to the library (maybe cool? probably not?) See these lines
• Audio Engine needs sound loaded callbacks. We should think about this along with how to move the sound loading into scratch-storage. Having loaded callbacks would allow a sound to be loaded and then played on demand, rather than what I'm doing now loading all the sounds in the library when the library is mounted. See these lines
• The VM needs an addSound function that also handles the loading of new sounds. See these lines Update The new scratch-storage integration will force the vm to have this functionality! Hooray!
• The library component is overfit for costumes/images. This is minor, but there is too much logic in the library component, it has logic for loading up images from md5's, etc. This should be backed out to the callers responsibility. I also had to add an addItemChosen callback for the actual select before the double click action.