New: radio buttons, "Blank" option

A couple of new features are out today, both related to multiple-choice columns.

First, the radio buttons. Up to now, we only supported dropdowns, and they're still the optimal UI element if you have a lot of options for the user to choose from: one can quickly pick an option by starting to type it, and the element takes up little space. But when you have just two or three options, radio buttons make for a better user experience because the user has to click only once to make a selection, and the whole list of options is visible right away. In the choices configuration dialog, we've added a checkbox that will tell the app to use a radio instead of a dropdown in the form view. That setting does not affect the table view, where we still always use dropdowns to save space.

Another new setting in the choices configuration dialog lets you add a Blank option to the list of choices. The reason we need that setting is that in some use-cases, you expect each record in the database to have a non-blank option selected, while in other use-cases, a blank cell is a state as legitimate as other states, and the user should have the ability to switch to it. A rule of a thumb is that you probably need the Blank option when you don't populate cells with a default value.