Refactor the workspace template installation process to add the extra pages and settings to a standard XE xar instead of bundling them in a workspace template xar

Right now, the workspace template consists of the standard XE xar + some Workspace related pages and settings. Because of this, the workspace template maven module can not be part of the platform since it is depending on the XE xar and thus causing a dependency cycle.

The initial solution was to handle the workspace template separately from the platform, manually building and publishing a build on extensions.xwiki.org for each XWiki new release. However, this is by far an optimal approach.

The better option is to no longer bundle the workspace specific items with an XE xar and let the admin specify the base template XAR he wants to use for the new workspace (of course, the recommendation is to use a default XE xar). Additional to the one-time base template xar specified by the admin, a second xar will be used (bundled with the workspace application) that contains only the extra workspace-specific pages that will be imported in the final workspace template at install time.

The idea is that everything is done with velocity at install time instead of using maven at build time. This comes at the expense of one extra install step that the Admin has to perform, but this does not impact regular users.