refactor: cut engine-to-UI include violations ahead of the liboakengine split

- slider DisplayType enums sink to node/sliderdisplaytype.h (canonical
  engine home); FloatSlider/RationalSlider alias them for compatibility
- DropWithoutSequenceBehavior enum sinks to common/dropworkflowbehavior.h
- Config errors now go through a registered ErrorHandler hook instead of
  QMessageBox with a MainWindow parent; the style default no longer
  depends on the UI style manager
- MainWindowLayoutInfo moves to node/project/serializer/ and its panel
  dependency is reduced to a plain std::map alias (PanelLayoutInfo),
  breaking the engine -> PanelWidget -> KDDockWidgets chain
- ProjectImportErrorDialog moves from task/ to dialog/projectimport/
- remove confirmed-redundant UI includes and give project.h/import.h/
  project.cpp the direct includes they were borrowing transitively
- project.h includes folder/sequence headers directly (it used both
  types in its own API all along)

app/main.cpp

@@ -341,6 +341,16 @@ int main(int argc, char *argv[])
 		a.reset(new QCoreApplication(argc, argv));
 	}
 
+	// Configuration errors are reported through a UI handler so the engine
+	// layer (config) never has to know about dialogs
+	olive::Config::set_error_handler(
+		[](const QString &title, const QString &message) {
+			QWidget *parent =
+				olive::Core::instance() ? olive::Core::instance()->main_window() :
+										  nullptr;
+			QMessageBox::critical(parent, title, message, QMessageBox::Ok);
+		});
+
 	olive::Config::load();
 	const QString graphics_backend =
 		olive::Config::current()[QStringLiteral("GraphicsBackend")]