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/dialog/markerproperties/markerpropertiesdialog.cpp

@@ -65,10 +65,10 @@ MarkerPropertiesDialog::MarkerPropertiesDialog(
 
 	if (markers.size() == 1) {
 		in_slider_->set_value(markers.front()->time().in());
-		in_slider_->set_display_type(RationalSlider::k_time);
+		in_slider_->set_display_type(slider::k_time);
 		in_slider_->set_timebase(timebase);
 		out_slider_->set_value(markers.front()->time().out());
-		out_slider_->set_display_type(RationalSlider::k_time);
+		out_slider_->set_display_type(slider::k_time);
 		out_slider_->set_timebase(timebase);
 	} else {
 		// Markers cannot be on the same time, so we disable setting time if multiple markers are selected