add example which registers and uses go custom elements

this example has an assert that checks wether or nor the finalizers ran correctly. If they don't then a mem leak will happen

examples/plugins/registered_elements/internal/custombin/element.go

@@ -0,0 +1,87 @@
+package custombin
+
+import (
+	"time"
+
+	"github.com/go-gst/go-glib/glib"
+	"github.com/go-gst/go-gst/examples/plugins/registered_elements/internal/common"
+	"github.com/go-gst/go-gst/gst"
+)
+
+type customBin struct {
+	// self    *gst.Bin
+	source1 *gst.Element
+	source2 *gst.Element
+	mixer   *gst.Element
+}
+
+// ClassInit is the place where you define pads and properties
+func (*customBin) ClassInit(klass *glib.ObjectClass) {
+	class := gst.ToElementClass(klass)
+	class.SetMetadata(
+		"custom test source",
+		"Src/Test",
+		"Demo source bin with volume",
+		"Wilhelm Bartel <bartel.wilhelm@gmail.com>",
+	)
+	class.AddPadTemplate(gst.NewPadTemplate(
+		"src",
+		gst.PadDirectionSource,
+		gst.PadPresenceAlways,
+		gst.NewCapsFromString("audio/x-raw,channels=2,rate=48000"),
+	))
+}
+
+// SetProperty gets called for every property. The id is the index in the slice defined above.
+func (s *customBin) SetProperty(self *glib.Object, id uint, value *glib.Value) {}
+
+// GetProperty is called to retrieve the value of the property at index `id` in the properties
+// slice provided at ClassInit.
+func (o *customBin) GetProperty(self *glib.Object, id uint) *glib.Value {
+	return nil
+}
+
+// New is called by the bindings to create a new instance of your go element. Use this to initialize channels, maps, etc.
+//
+// Think of New like the constructor of your struct
+func (*customBin) New() glib.GoObjectSubclass {
+	return &customBin{}
+}
+
+// InstanceInit should initialize the element. Keep in mind that the properties are not yet present. When this is called.
+func (s *customBin) InstanceInit(instance *glib.Object) {
+	self := gst.ToGstBin(instance)
+
+	s.source1 = common.Must(gst.NewElementWithProperties("gocustomsrc", map[string]interface{}{
+		"duration": int64(5 * time.Second),
+	}))
+	s.source2 = common.Must(gst.NewElementWithProperties("gocustomsrc", map[string]interface{}{
+		"duration": int64(10 * time.Second),
+	}))
+
+	s.mixer = common.Must(gst.NewElement("audiomixer"))
+
+	klass := instance.Class()
+	class := gst.ToElementClass(klass)
+
+	self.AddMany(
+		s.source1,
+		s.source2,
+		s.mixer,
+	)
+
+	srcpad := s.mixer.GetStaticPad("src")
+
+	ghostpad := gst.NewGhostPadFromTemplate("src", srcpad, class.GetPadTemplate("src"))
+
+	s.source1.Link(s.mixer)
+	s.source2.Link(s.mixer)
+
+	self.AddPad(ghostpad.Pad)
+}
+
+func (s *customBin) Constructed(o *glib.Object) {}
+
+func (s *customBin) Finalize(o *glib.Object) {
+	common.FinalizersCalled++
+}