Introduce a new HtmlVariableListWidget that is similar to
VariableListWidget but for variables with HTML contents.
Update the collectors that use the existing VariableListWidget to use
DebugBarVarDumper to dump the variables using the VarDumper HtmlDumper.
Because many debug bar users may not yet support inline static assets,
default to use the old VariableListWidget for now.
Updated collectors:
* ConfigCollector
* RequestDataCollector
