[Property Editor] Label should always be at the top of the input

[elliette]

Work towards #1948

Previously, if a text input was empty (e.g. an empty string) the label text would display in the input box itself. This meant that most of the labels would be at the top of the input boxes, while others would show up in the input box:

This change makes sure the labels always stay at the top of the input box:

Note: I'm still trying to decide how/if we want to try to show the surrounding quotes for string inputs. For example above, that would mean showing '' in the input box instead of nothing.