Add the Views display as an option

Currently, we always use the Master display for building the view when we want to execute an autocomplete query for one. This can lead to wrong settings being used.
The easiest way to fix this is to just let users select which display should be used. This isn't ideal, but determining the right display would be very tricky, and would also have less flexibility in some aspects.

One other thing the patch in comment 1 addresses is to document the lack of support for contextual filters / arguments. As views can receive their arguments from virtually anywhere, and the context in which we execute the autocomplete query is very different from the one in which the corresponding view is normally executed, there is really no way to reliably support any portion of this functionality. Therefore, I opted to just clearly document this shortcoming and encourage users to use a display without contextual filters as the base for the autocomplete query. (The problem here is, also, that the view might receive wrong arguments and/or return no results at all due to validation errors – even if just skipping the arguments would be unproblematic.)