add type (or kind) of textfield

It would be nice to have the ability to control input fields a bit, kind of like `secure` for password fields, but more general, for these types (sort of like html5 field types).

- arbitrary strings (default)
- password (the current `secure` inside `style`)
- email (brings up the email keyboard)
- integer (brings up the numeric keyboard)

perhaps one day down the road you could have touch id and other kinds.

not a high priority, since there is a decent alternative. just a nice to have.

Start by locating the textfield implementation and the existing `secure` style handling. Compare the requested arbitrary, password, email, and integer behaviors with the current input configuration and determine how keyboard selection is exposed. Done means the supported kinds work while arbitrary strings remain the default; no tests or files are named in the issue.

Written by the indexing model from the issue text.