Modified CompositeJsTagHandlerPlugin to proxy through to ContentPlugin
instances rather than TagHandlerPlugin instances, halving the number of
plug-ins developers have to create, and removing an unnecesarry
code-path that had to be separately tested, but often wasn't. Also,
modified I18NContentPlugin so that it is no longer a part of JavaScript
bundle -- this was only happenning if single files were used, which is
why we never noticed it.
Removed now unnecesarry tag-handler plug-ins.
Removed now defunct TagHandlePlugin.getGroupName() method.
TagHandlerPlugin no longer extends OrderedPlugin, since ordering was
only necesarry to support the composite behaviour we no longer have.
Removed locale support from CompositeJsTagHandlerPlugin and
CompositeJsContentPlugin, since the JavaScript bundles don't need to
differentiate based on locale. This is the final fix for #532.
Merge pull request #644 from BladeRunnerJS/simplify-composite-plugins

Simplify composite plugins