add Windows taskbar control buttons (TODO: icons)

backend/windows/smtc_cfuncs.go

@@ -0,0 +1,16 @@
+//go:build windows
+
+package windows
+
+/*
+void btn_callback_cgo(int in) {
+	void btnCallback(int);
+	btnCallback(in);
+}
+
+void seek_callback_cgo(int in) {
+	void seekCallback(int);
+	seekCallback(in);
+}
+*/
+import "C"