Forms 2.0

Overview:

• Major rework of forms with per-field validation, messages, and more fluid metadata parsing.
• Added a message to the Select dialog.
• Configurable affixes to sliders. This allows you to have e.g. a $ prefix to the value of a slider.
• Auto implementation of State for dialogs. This allows us to reuse state::Signal for dialogs, and
  dialog::Signal is therefore removed.
• Bumped Ratatui version to 0.27.

Forms 2.0!

You can now specify per-field validation of forms using control statements. Each control statement declares
an error condition as a boolean function over the value of the field, and an error message. If the error
condition triggers, the name of the offending field turns red, and the erorr message is shown if the user
attempts to submit the form.

Form metadata can now also be given in any order. This allows us to add more optional metadata without
requiring that all previous optional metadata is given (e.g. how default parameters to functions work in many
languages), and should be more intuitive to use. Using this, we've added a message that can be shown above
the fields when the form is displayed, which may be useful for explaining the context and effects of a form
to the user.

See the form macro documentation for more information.