Improved functionality

Changed function name from load_plugin_textdomain and add_plugin_... to load_core_textdomain and add_core_... respectively for compatibility reasons
Added variable and function to registere plugins to provide means to check whether the core is still necessary after plugins have been uninstalled
Added a function to include subpages of plugins using the MBV Core Library
Created pages to wrap plugin content and display a submenu for each plugin, if required
Added some icons and css
Created a class for language / localization handling using static functions (for now)