API: Treedropdownfield showsearch default true, provide better ui

* Provide search option to 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

![26_08_13_12_20_pm](https://f.cloud.github.com/assets/984753/1023372/f4f38e88-0de5-11e3-990e-328a8403c86d.png)