Allow adding a custom side menu to EditorFileDialog

doc/classes/EditorFileDialog.xml

@@ -19,6 +19,14 @@
 				For example, a [param filter] of [code]"*.tscn, *.scn"[/code] and a [param description] of [code]"Scenes"[/code] results in filter text "Scenes (*.tscn, *.scn)".
 			</description>
 		</method>
+		<method name="add_side_menu">
+			<return type="void" />
+			<param index="0" name="menu" type="Control" />
+			<param index="1" name="title" type="String" default="&quot;&quot;" />
+			<description>
+				Adds the given [param menu] to the side of the file dialog with the given [param title] text on top. Only one side menu is allowed.
+			</description>
+		</method>
 		<method name="clear_filters">
 			<return type="void" />
 			<description>

editor/gui/editor_file_dialog.cpp

@@ -1619,6 +1619,7 @@ void EditorFileDialog::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("_thumbnail_result"), &EditorFileDialog::_thumbnail_result);
 	ClassDB::bind_method(D_METHOD("set_disable_overwrite_warning", "disable"), &EditorFileDialog::set_disable_overwrite_warning);
 	ClassDB::bind_method(D_METHOD("is_overwrite_warning_disabled"), &EditorFileDialog::is_overwrite_warning_disabled);
+	ClassDB::bind_method(D_METHOD("add_side_menu", "menu", "title"), &EditorFileDialog::add_side_menu, DEFVAL(""));
 
 	ClassDB::bind_method(D_METHOD("invalidate"), &EditorFileDialog::invalidate);
 
@@ -1712,6 +1713,25 @@ bool EditorFileDialog::are_previews_enabled() {
 	return previews_enabled;
 }
 
+void EditorFileDialog::add_side_menu(Control *p_menu, const String &p_title) {
+	// HSplitContainer has 3 children at maximum capacity, 1 of them is the SplitContainerDragger.
+	ERR_FAIL_COND_MSG(body_hsplit->get_child_count() > 2, "EditorFileDialog: Only one side menu can be added.");
+	// Everything for the side menu goes inside of a VBoxContainer.
+	VBoxContainer *side_vbox = memnew(VBoxContainer);
+	side_vbox->set_h_size_flags(Control::SIZE_EXPAND_FILL);
+	side_vbox->set_stretch_ratio(0.5);
+	body_hsplit->add_child(side_vbox);
+	// Add a Label to the VBoxContainer.
+	if (!p_title.is_empty()) {
+		Label *title_label = memnew(Label(p_title));
+		title_label->set_theme_type_variation("HeaderSmall");
+		side_vbox->add_child(title_label);
+	}
+	// Add the given menu to the VBoxContainer.
+	p_menu->set_v_size_flags(Control::SIZE_EXPAND_FILL);
+	side_vbox->add_child(p_menu);
+}
+
 EditorFileDialog::EditorFileDialog() {
 	show_hidden_files = default_show_hidden_files;
 	display_mode = default_display_mode;
@@ -1739,6 +1759,7 @@ EditorFileDialog::EditorFileDialog() {
 	}
 
 	HBoxContainer *pathhb = memnew(HBoxContainer);
+	vbc->add_child(pathhb);
 
 	dir_prev = memnew(Button);
 	dir_prev->set_theme_type_variation("FlatButton");
@@ -1826,11 +1847,13 @@ EditorFileDialog::EditorFileDialog() {
 	makedir->connect("pressed", callable_mp(this, &EditorFileDialog::_make_dir));
 	pathhb->add_child(makedir);
 
-	list_hb = memnew(HSplitContainer);
+	body_hsplit = memnew(HSplitContainer);
+	body_hsplit->set_v_size_flags(Control::SIZE_EXPAND_FILL);
+	vbc->add_child(body_hsplit);
 
-	vbc->add_child(pathhb);
+	list_hb = memnew(HSplitContainer);
-	vbc->add_child(list_hb);
+	list_hb->set_h_size_flags(Control::SIZE_EXPAND_FILL);
-	list_hb->set_v_size_flags(Control::SIZE_EXPAND_FILL);
+	body_hsplit->add_child(list_hb);
 
 	VSplitContainer *vsc = memnew(VSplitContainer);
 	list_hb->add_child(vsc);

editor/gui/editor_file_dialog.h

@@ -103,6 +103,7 @@ private:
 	PopupMenu *item_menu = nullptr;
 	TextureRect *preview = nullptr;
 	VBoxContainer *preview_vb = nullptr;
+	HSplitContainer *body_hsplit = nullptr;
 	HSplitContainer *list_hb = nullptr;
 	HBoxContainer *file_box = nullptr;
 	LineEdit *file = nullptr;
@@ -282,6 +283,8 @@ public:
 	void set_previews_enabled(bool p_enabled);
 	bool are_previews_enabled();
 
+	void add_side_menu(Control *p_menu, const String &p_title = "");
+
 	EditorFileDialog();
 	~EditorFileDialog();
 };