add unsafe_set!(::Textbox, ::String)

[bjarthur]

Description

a return value of nothing is clearly wrong for this do-block as the lifted observable is a Tuple of Points.
i came across this error when manually setting displayed_string in an app of mine. happy to add a test if someone suggests how to do so.

EDIT: a 2nd commit now adds an unexported unsafe_set!, which like the extant set! method sets the displayed_string but does not throw an exception if the string does not pass the validator function. the use case is to highlight the portion of the string that does not pass the validator by replacing it with the invalid portion. was the most simple UI for an app of mine.

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• Added unit tests for new algorithms, conversion methods, etc.
• Added an entry in CHANGELOG.md (for new features and breaking changes)

[bjarthur]

ready for review!