MantisCoreFormatting: Fix single-line fields html tags processing

Methods text() and formatted() Plugin sent a hardcoded 'true' parameter
to string_restore_valid_html_tags(), instead of using the method's
$p_multiline parameter.

Fixes #16342
MantisCoreFormatting: phpDoc corrections and improvements
MantisCoreFormatting: Remove code duplication

Method formatted() now calls text() instead of duplicating plain text
processing code

Bump plugin version to 1.0b

Fixes #16348