New {{raw}} macro to generate a RawBlock

Right now it's not possible in syntaxes to generate a RawBlock.

Use case example:

• The LaTeX extension provides templates which can have extension points
• These UIX must generate latex/1.0 content
• The syntax of extensions points is the syntax of the wiki page holding the UIExtensionClass xobject.
• That page cannot be in "latex/1.0" syntax since we don't have a parser for that syntax id.
• Thus we have to use for example "xwiki/2.1" syntax to, somehow, generate a RawBlock with a target syntax of "latex/1.0".

Usage example:

{{raw syntax="latex/1.0"...{{/raw}}