qtdemux: rework taglist handling
qtdemux: rework taglist handling

Keep taglist around during element existance to avoid having to
create it at different places before usage. Makes code simpler to handle.

https://bugzilla.gnome.org/show_bug.cgi?id=766177

GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good