Added dropdown binding editor for easy assignment

[tomsmeltzer]

Primary goal is to bring the dropdown binding up to near the usability standards of the other bindings with the addition of a binding editor.

Because the dropdown binding has a very specific use case, the destination type is hardcoded. The option to use an adapter exists for the dropdown options, for which I have included a string list to string array adapter. Without an adapter it will accept a string array property.

Also included is an adapter for the inverse direction (string array to string list). There is likely no purpose for this adapter in the context of dropdowns, but it is conceivable that developers may find utility in it in other ways.

There is no current adapter options object that this adapter will accept since I can't think of any optional parameters that would be required.

Note that this commit has left boldening of values changed on prefabs as non-functional.