Improve UI of over-screen tab search

[jchang504]

I wrote 004ff6c pretty fast to get #27 done so Roger could work on some stuff without worrying about our background and content script diverging a lot and having a messy merge. But there are some UI improvements that I didn't have time to figure out, mostly related to weird stuff happening with the tab search filtering (maybe related to fuse):

1. Even though I call populate() when opening the tab search, no results show until the user inputs something; this differs from the original popup tab search behavior and isn't horrible, but it would be better if it just showed all the tabs at the start.
2. The fuse fuzzy search seems to be keeping some state and "remembering" the previous search, as even after backspacing and deleting all the characters, the results are different (related to previously typed characters) than a plain list of all the tabs.

[jchang504]

Also, would be nice to have a shrinking animation to mirror the expanding animation when you close search with the hold key.