refactor Gstreamer class to be more modular
    
      * Gstreamer Bins provide their own fake sources/sinks when disabled
      * removed input tees, queues, and sinks from CompositorBin
      * CompositeSink is now much larger single preview
      * toggle devices, preview, and output via GUI requires restart (issue #21)
      * added Gstreamer::MakeLctvUrl(), Gstreamer::MakeVideoCapsString()
      * removed Gstreamer::FullscreenSink, Gstreamer::OverlaySink, Gstreamer::CompositeSink
      * added Gstreamer::WindowHandle
      * removed Gstreamer::MakeRequestSrcPad(), Gstreamer::MakeRequestSinkPad(), Gstreamer::MakeRequestGhostPad()
      * removed AvCaster::ToggleOutput(), AvCaster::ToggleInterstitial(), AvCaster::ToggleScreencap(),
                AvCaster::ToggleCamera(), AvCaster::ToggleText(), AvCaster::TogglePreview()
      * added Gstreamer::ReconfigureOutput(), Gstreamer::ReconfigureInterstitial(), Gstreamer::ReconfigureScreencap()
              Gstreamer::ReconfigureCamera(), Gstreamer::ReconfigureText(), Gstreamer::ReconfigurePreview()
      * renamed Config->outputStreamCombo() to Config->outputSinkCombo()
      * renamed PresetSeed->containerIdx to PresetSeed->muxerIdx and PresetSeed->streamIdx to PresetSeed->sinkIdx