It is a plugin with three buttons which displays the current shuffle and loop mode and lets you change it with a mouse click. The new third button is used to add new playback modes ("Keep Album", "Keep Artist", "Top Rated", "Selection", "Playlist" deactivates the plugin). To compile the plugin you need to copy the files deadbeef.h and gtkui_api.h from the deadbeef directory.
Copy the compiled plugin to the plugin folder (~/.local/lib/deadbeef/) and restart DeadDBeeF, then add the plugin to the gui.