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)
Merge pull request #2364 from adrexia/tree-dropdown-search

API: Treedropdownfield showsearch default true, provide better ui