Add terminal widget to controllers

[Choreas]

Hi :) This adds read-only terminals to controllers. Terminal widgets can be created by converting a button from its edit view. Terminals only have settable height and width.

I wasn't able to easily split ControlElement into Buttons and Terminals, because I haven't worked with react before. This would have made it more clean. If you want to pull this but can't because of this or other issues, let me know.