Port choices in request widgets to GtkDropDown

This also fixes the actual response as well.

Unlike before, the choice field no longer differentiates between "many" choices/compact choices vs "few", and just always uses a drop down.

Testing Done:
Opened a choice request, as well as a request field with a choice in it. Confirmed that selected choices were returned from the dialog.

Reviewed at https://reviews.imfreedom.org/r/2290/