Partial Fix for 1.5

[penguin0616]

The Widgets.TextAreaScrollable method got moved to LudeonTK.DevGUI.

Fixing the reference makes the tile highlighting works, but the scrollable of selected items is blank. I made do without it.

The method itself doesn't seem to have significantly changed beyond the readOnly interpretation.

This was my first time modding with RimWorld, so hopefully I didn't butcher any of the non-code files.