Bug 838033 - Music search

/!\ This isn't ready for merging yet! /!\

f? @dominickuo: What do you think of this so far? Specifically, my changes from `changeMode` to `ModeManager`. I did that so we could have a stack of modes, since the flow is a bit more complicated now that we have search.

I'm also curious if you can think of a good way to put the search widget in all modes and have scrolling work as it should (i.e. scroll up when you're at the top to open the search). Here's the spec for reference: https://www.dropbox.com/s/yepxm9hogne8apy/music-search.pdf