API: Treedropdownfield showsearch default true, provide better ui

* Set search option true on treedropdown fields by default, to provide a fallback solution when trees fail to render (too many children errors)

* Provide better indication/more meaningful styling to search (match chosen styles for consistency)

Tested in Chrome, Firefox, IE8 and IE9

![tree](https://f.cloud.github.com/assets/984753/1023378/a7d3df8a-0de6-11e3-8467-11f1e4d539c0.png)