FIX: Allow TreeDropdownField to open upwards (fixes #2188)

Tested in FF, Chrome, IE8 (no copy of IE9/10 to check on)

![screen shot 2013-10-23 at 13 28 46](https://f.cloud.github.com/assets/1655548/1389730/2b3dd526-3bdf-11e3-967b-44ad3ebdfcb4.png)

IE8:
![screen shot 2013-10-23 at 13 31 21](https://f.cloud.github.com/assets/1655548/1389731/2ca23a4c-3bdf-11e3-9f0b-e36baf5d1c97.png)