PluginFileMerger support for multi-line property values

- Now only parsing property keys from the first line of a property key/value pair. Previously an embedded '=' within a multi-line property value would be treated as another key/value pair.
- When commenting out overridden properties, all lines of a multi-value property are commented out.
Merge pull request #2039 from signed8bit/handle-multiline-props

PluginFileMerger support for multi-line property values fixes #2042