Refactored secret string handling. Now the plaintext is always held in the
same buffer for both secret and non-secret strings, and a string of
asterisks is generated on the fly for display when necessary.
The motivation for this change is that some keyboard input (e.g.
backspace, delete) was not handled properly for secret strings, and it
made more sense to keep input handling uniform, and instead customise
rendering (rather than have two ways of handling every type of keyboard
input).
git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@50708 fb15a70f-31f2-0310-bbcc-cdcc74a49acc