Add validation functions for textArea and dropdown

[srid]

The current validationInput only works with inputElement. This PR creates two similar functions to work with textArea and dropdown.

Unlike the other two input types, dropdown input's values can be of arbitrary type, v, instead of necessarily being Text. To accommodate this ValidationConfig type now takes an extra type parameter, v, that specifies this type. For validationInput and validationTextArea this is Text, but for validationDropdown it is the user that will provide this type.

[srid]

This PR now targets the develop branch.