Expose FileSystemDock to the scripting API and fixed signals

Fixed FileSystemDock's `file_removed` and `folder_removed` signals not being
emitted because the emitted was using the wrong signal name.

(cherry picked from commit fe0b783e70)

editor/editor_node.cpp

@@ -3673,6 +3673,7 @@ void EditorNode::register_editor_types() {
 	ClassDB::register_class<ScriptCreateDialog>();
 	ClassDB::register_class<EditorFeatureProfile>();
 	ClassDB::register_class<EditorSpinSlider>();
+	ClassDB::register_virtual_class<FileSystemDock>();
 
 	// FIXME: Is this stuff obsolete, or should it be ported to new APIs?
 	ClassDB::register_class<EditorScenePostImport>();