Implemented drag-to-delete functionality.

When keywords are picked up from a synth pad (not when they're taken from the sidebar on the left), a 'delete keyword' div will now appear above the synth. If the keyword is dropped onto it, the keyword will go away. Since the sound-playing event listener connecting the keyword to its synth pad is removed as soon as dragging begins and is only reinstated when the keyword reverts back to the pad, this should effectively remove the listener.