fix object reference field background being transparent when empty

[zivmaor]

Description

Fix #1383. The problem was that the hint for when the object reference field is empty was set as the background of the textbox via a label, and thus the background became transparant. I don't know how this can be fixed with the hint still being in the background, but I fixed it by making the hint the text of the textbox. This should be ok in this context because the textbox is readonly. If there is a way to do this with the hint still being in the background then this PR can be rejected.

Caveats

I used a valueConverter in weird way to make this work. If that is not ok I can make it work without that via making the UndertaleObjectReference use INotifyPropertyChanged