Taxon autocomplete search tab-completion

[JWCook]

Follow-up from #4 and #20 . The taxon search completion is still a little clunky.

• Format suggestion text to complete current input, e.g: Brown mar|morated stink bug
• Display autocomplete suggestion text as the user types
• Enable completion by pressing Tab (to complete search input, but not select the item yet)
• Cycle through additional matches by pressing Tab again or with arrow keys
• Asynchronously (re)load dropdown results
• Draw dropdown box on top of other widgets instead of expanding to push them down (requires FloatLayout)
• Make dropdown items less ugly... needs some formatting