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/ui/colorcoding.h

@@ -0,0 +1,72 @@
+/***
+
+  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/>.
+
+***/
+
+#ifndef OAK_COLORCODING_H
+#define OAK_COLORCODING_H
+
+#include <olive/core/core.h>
+#include <QObject>
+
+namespace olive
+{
+
+using namespace core;
+
+class ColorCoding : public QObject {
+	Q_OBJECT
+public:
+	enum Code {
+		k_red,
+		k_maroon,
+		k_orange,
+		k_brown,
+		k_yellow,
+		k_olive,
+		k_lime,
+		k_green,
+		k_cyan,
+		k_teal,
+		k_blue,
+		k_navy,
+		k_pink,
+		k_purple,
+		k_silver,
+		k_gray
+	};
+
+	static QString get_color_name(int c);
+
+	static Color get_color(int c);
+
+	static Qt::GlobalColor get_ui_selector_color(const Color &c);
+
+	static const QVector<Color> &standard_colors()
+	{
+		return colors;
+	}
+
+private:
+	static QVector<Color> colors;
+};
+
+}
+
+#endif // OAK_COLORCODING_H