move new_playlist Lua script from mixbus and add some garbage collection

scripts/new_playlist.lua

@@ -0,0 +1,17 @@
+ardour {
+	["type"]    = "EditorAction",
+	name        = "New Playlist",
+	license     = "MIT",
+	author      = "Ardour Lua Taskforce",
+	description = [[Prompts and builds a new playlist for every track in the session.]]
+}
+
+function factory () return function ()
+
+	for r in Session:get_tracks():iter() do
+		local rtav = Editor:rtav_from_route(r) -- lookup RTAV
+		Editor:new_playlists(rtav:to_timeaxisview())
+	end
+
+collectgarbage()
+end end