BUG: Treedropdowns broken inside togglecomposite fields (fixes #1945)

* Removed positioning javascript from treedropdownfield
* Set parent to be relatively positioned and overflow to visible

I have tested with large and small sitetrees, inside a gridfield, in the files area, inside a toggle composite, and at the very bottom of a tab (after scrolling: n.b. it was originally broken here too). 

Browsers tested:
* Firefox
* Chrome
* IE9
* IE8 (this browser does have a slight issues, as it doesn't resize the toggle composite field to fit the treedropdown field. This fix doesn't change anything on that front. )