Makefile.am, main.js, mainWindow.js, waveform.js: Add signals so pipeline is...

Makefile.am, main.js, mainWindow.js, waveform.js: Add signals so pipeline is set to null before memory is released

Gstreamer was warning that memory was being freed before the pipeline elements were set to null. This fixes it using the method suggested by Nicolas Dufresne
See https://bugzilla.gnome.org/show_bug.cgi?id=723209