Feature: Added zip extraction to filetree-menu

Based on the forum discussion here: http://forums.modx.com/thread/84971/unzip-function-for-file-manager#dis-post-497458

This adds a filetree context menu item "Extract file" for .zip files (zip only for now), the PR includes also an additional permission and de + en lexicon entries plus all necessary modifications in the core to make it work.

Credits also go to Susan Ottwell for providing the right idea and approach to get this running!