Reduce number of dependencies drawn transitively by the xwiki-velocity module

Namely split the module into 2:

• xwiki-velocity-default
• xwiki-velocity-webapp (implementation of VelocityEngine and VelocityConfiguration with hint = "webapp", this is to use use velocity with the WebappResourceLoader loading templates from the webapp's root dir)

This allows other modules to only depend on what they need and thus reduce the number of transitive dependencies drawn

Future: find a way to add xwiki-velocity-management and externalize registering an MBean per Velocity Engine