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/node/block/subtitle/subtitle.cpp

@@ -0,0 +1,74 @@
+/***
+
+  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 "subtitle.h"
+
+namespace olive
+{
+
+#define super ClipBlock
+
+const QString SubtitleBlock::k_text_in = QStringLiteral("text_in");
+
+SubtitleBlock::SubtitleBlock()
+{
+	add_input(k_text_in, NodeValue::k_text,
+			 InputFlags(k_input_flag_not_connectable | k_input_flag_not_keyframable));
+
+	set_input_flag(k_buffer_in, k_input_flag_hidden);
+	set_input_flag(k_length_input, k_input_flag_hidden);
+	set_input_flag(k_media_in_input, k_input_flag_hidden);
+	set_input_flag(k_speed_input, k_input_flag_hidden);
+	set_input_flag(k_reverse_input, k_input_flag_hidden);
+	set_input_flag(k_maintain_audio_pitch_input, k_input_flag_hidden);
+
+	// Undo block flag that hides in param view
+	set_flag(k_dont_show_in_param_view, false);
+}
+
+QString SubtitleBlock::name() const
+{
+	if (get_text().isEmpty()) {
+		return tr("Subtitle");
+	} else {
+		return get_text();
+	}
+}
+
+QString SubtitleBlock::id() const
+{
+	return QStringLiteral("org.olivevideoeditor.Olive.subtitle");
+}
+
+QString SubtitleBlock::description() const
+{
+	return tr(
+		"A time-based node representing a single subtitle element for a certain period of time.");
+}
+
+void SubtitleBlock::retranslate()
+{
+	super::retranslate();
+
+	set_input_name(k_text_in, tr("Text"));
+}
+
+}