feature request: required attribute and empty choice for `dropdown`

[horriblename]

I need a single-selection dropdown that makes sure an option is selected before submitting the form: https://git.uni-due.de/fmi/autotool-dev/-/merge_requests/70#note_250192

This is usually done like this in html:

<select required>
  <option value="">---</option>
  <option value="1">One</option>
  ...

The important parts being required attribute and an option with value="" (has to be the first element)

Right now required can be set easily with fsAttrs, but value is "hard-coded" to [1..] in

flex-tasks/flex-tasks/src/FlexTask/Generic/FormInternal.hs

Line 488 in 86e93a5

formifyImplementation = renderNextSingleChoiceField (`zip` [1..]) . (=<<) getAnswer

I'm thinking we can check for the required attribute in renderNextSingleChoiceField and, if it exists, add an additional option with value=""

(Of course there are probably other ways to implement this)

[patritzenfeld]

I've decided to add this explicitly via the Formify instance of SingleChoiceSelection, so you do not manually need to add attributes. There's now also a Maybe SingleChoiceSelection instance.

For radio buttons, the SingleChoiceSelection instance is unchanged, while the Maybe SingleChoiceSelection instance just adds a None button.

For dropdowns, the SingleChoiceSelection instance now forces an active input via your suggested pattern. Maybe SingleChoiceSelection retains the previous behaviour, but also directly provides the None option we usually added manually.

I advice you to wait until I've adjusted the existing tasks in Autotool before you work with this for new tasks. (just so you don't have to fix all of the compilation problems yourself)