build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

engine/undo/undocommand.cpp

@@ -0,0 +1,91 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "undocommand.h"
+
+#include "node/project.h"
+
+namespace olive
+{
+
+void MultiUndoCommand::redo()
+{
+	for (auto it = children_.cbegin(); it != children_.cend(); it++) {
+		(*it)->redo_and_set_modified();
+	}
+}
+
+void MultiUndoCommand::undo()
+{
+	for (auto it = children_.crbegin(); it != children_.crend(); it++) {
+		(*it)->undo_and_set_modified();
+	}
+}
+
+UndoCommand::UndoCommand()
+{
+	prepared_ = false;
+	done_ = false;
+}
+
+void UndoCommand::redo_and_set_modified()
+{
+	project_ = get_relevant_project();
+
+	redo_now();
+
+	if (project_) {
+		modified_ = project_->is_modified();
+		project_->set_modified(true);
+	}
+}
+
+void UndoCommand::undo_and_set_modified()
+{
+	undo_now();
+
+	if (project_) {
+		project_->set_modified(modified_);
+	}
+}
+
+void UndoCommand::redo_now()
+{
+	if (!done_) {
+		if (!prepared_) {
+			prepare();
+			prepared_ = true;
+		}
+
+		redo();
+		done_ = true;
+	}
+}
+
+void UndoCommand::undo_now()
+{
+	if (done_) {
+		undo();
+		done_ = false;
+	}
+}
+
+}