NEW: Add ReadonlyField::setIncludeHiddenField()

The new config setter restores the 2.4 behaviour of including <input type="hidden"> with a field. Although as a default, this option has security flaws; it is useful in a few circumstances and, if nothing else, is handy to make upgrading sites easier.
BUGFIX: Don't clear form messages unless forTemplate() is actually called.
BUGFIX: Clear session-stored form data as well as form error message.
Merge pull request #1082 from sminnee/form-improvements

Form improvements