UiRegion: respect inner node's preferred size
UiRegion: respect inner node's preferred size

By default, the minHeight of Regions is 0. In the case of UiRegion, this
leads to their inner nodes getting squished vertically. A minHeight of 0
is thus not a very sane default.

Let's use a more sane default by setting UiRegion's default minHeight to
their inner node's preferred size.

CS2103AUG2016-T11-C4/main,nightism/addressbook-level4,CS2103AUG2016-T11-C4/main,snehasp13/main,snehasp13/main,nightism/addressbook-level4