Move SimpleExport to libardour, add Lua bindings

This allows to export a session from the commandline tool

```Lua
AudioEngine:set_backend("None (Dummy)", "", "")
s = load_session ("/path/to/session", "snapshot")
assert (s);
e = s:simple_export()
assert (e:check_outputs ())
e:run_export ()
e = nil
```

libs/ardour/lua_api.cc

@@ -37,6 +37,7 @@
 #include "ardour/plugin_manager.h"
 #include "ardour/readable.h"
 #include "ardour/region_factory.h"
+#include "ardour/simple_export.h"
 #include "ardour/source_factory.h"
 
 #include "LuaBridge/LuaBridge.h"
@@ -534,6 +535,16 @@ ARDOUR::LuaAPI::wait_for_process_callback (size_t n_cycles, int64_t timeout_ms)
 	return true;
 }
 
+int
+ARDOUR::LuaAPI::simple_export (lua_State* L)
+{
+	Session* const s = luabridge::Userdata::get <Session> (L, 1, false);
+	void* ptr = luabridge::UserdataValue<SimpleExport>::place (L);
+	SimpleExport* se = new (ptr) SimpleExport ();
+	se->set_session (s);
+	return 1;
+}
+
 void
 ARDOUR::LuaAPI::segfault ()
 {