Allow configuring the list of syntaxes a given wiki supports

Implementation details:

• Add a property in xwiki.cfg to enable different syntaxes:

# [Since 1.6RC1] Defines the list of supported syntaxes
# Available syntaxes are: 
#    xwiki/1.0, xwiki/2.0, confluence/1.0, jspwiki/1.0, creole/1.0, 
#    mediawiki/1.0, xhtml/1.0, twiki/1.0
xwiki.rendering.syntaxes = xwiki/1.0, xwiki/2.0

• It defaults to XWiki Syntax 1.0 and 2.0 (because the other syntaxes are not quite ready)
• Add a new XWiki API: XWiki.getConfiguredSyntaxes()
• Any user will be able to see the list of configured syntaxes in wiki edit mode