Improvements to the plugin system

 * Rendering the linklist has been moved to a separate function (to allow greater customisation if the need arises)
 * A PageBuilder is now created every time we render the page. Although it is not always page settings, like we'll do with plugins. Also, this removes the potential errors when forgetting to create a pageBuilder and so on.
 * We now build a list of plugins for each location (for now: toolbar, linklist, footer) so that RTPL has no issues rendering them
 * Simplified plugins templates. Linklist templates can now access the link info through the  variable, while toolbar and footer can access global settings via .
 * This means we don't have to create empty files anymore to prevent raintpl from crashing