Introduce mixin to configure the appPath of a wicket plugin

Current situation

The name of the plugin class ( *Perspective) is used to infer the name of the menu item. Also the list of menu items depends on the value of wicket.id. In the new role based authorization the authorization on the plugin is not the criteria for showing a perspective.

Proposed solution

Introduce a mixin frontend:navigationitem that can be added to a plugin. When a plugin has this mixin it indicates that it represents a navigation item. The mixin requires an appPath property that will be used to construct a navItem. If a plugin with such a mixin should be returned as a navItem usually depends on the userrole the logged in user has. Therefore, the plugin should, in addition to the mixin also have the proper hipposys:userrole. The plugin is a navItem only when the user has the correct user role.