Split css file

The style.css file in package-skin has a lot of different types of classes. These should be split into

• generic classes
  these can be used by any plugin to get a consistent look & feel (hippoecm-title, hippoecm-dialog, ...)

• plugin specific classes
  specific to the plugin; style file is in the same package as plugin

The use of prefixes (e.g. hippoecm-) should prevent collisions with other markup providers (Wicket, Xinha, Yui).