Working Context Item Addons

Added rudimentary logging and error handling in ContextItemAddon Ctor

Added copy_if to Util.h

Added String 'Context Item'
Let context addon access their settings in the visibility expression via Addon.HasSetting()... still stuff to do

Context items can now have addon.hasSetting... this also reacts to setting changes
GUIContextManager should now be hooked up everywhere (not just the sources views)... but it seems kind of strange that GUIMediaWindows is the point to add global context items and not GUIDialogContextMenu ;)

actually handle the oncontextmenu events
Context Item Addons can now be added to the addon browser

Context Item Addons now available in Video Playlists

Context Item Addons now available in Music Playlists

Context Item Addons now available in Favorites Dialog
some rudimentary work in order to pass the selected GUIListItem to the context item addon
fixed some wrong #include paths.... wondering why it worked before?!!