default plugin app: let user pick up arbitrary sample audio

[atsushieno]

A remaining task from #153 - the new app UI landed, but we still do not let user pick up an arbitrary audio file to test with the plugin.

I'm kind of indecisive of the actual UI design though - whether we let an app organize the samples and interact with each plugin app (complicated if the user just uses one plugin app), or each single app has storage for the loaded samples (user needs to store them on every app).