Merging the pack into ItemsAdder

• Fixed: with resource-pack.mode: external, ItemsAdder had nothing it could merge, because it merges folders and MC GUI only wrote pack.zip. The pack is now also unpacked into plugins/MCGUI/pack/. Add MCGUI/pack under resource-pack.zip.merge_other_plugins_resourcepacks_folders in ItemsAdder's config.yml and run /iazip.
• Fixed: with the pack merged into another plugin's, pages refused to open with "You have not loaded the resource pack that screens need" when that plugin sent its pack while the player was joining, as ItemsAdder does. A merged pack now counts as loaded unless the player declined it or it failed.
• The console now says at startup where to merge the pack from in external mode.