Need to be able to change a Plain Text Property to Text

Summary: Alexx created some Plain Text Properties, which really ought to be Text instead. But the Editor doesn't seem to work right when changing a Property's Type.

I'm probably going to do a multi-level fix here. First, I'm going to make it illegal to do most Type changes, because they will break things. However, the Text Types are more or less fungible, so I'm going to allow this. I need to figure out why it isn't displaying the change properly. (It is changing the Property in the DB, and on the next reload the change takes effect. I don't know why it isn't showing the change immediately, though.)

Justin: okay, it now works. It is specifically legal now to change a Property between any of the three textual Types (Text, Large Text and Plain Text). Nothing else is allowed for now, but those three all serialize identically, so it's a relatively safe fix, I believe. Please tell me if you have any issues.