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/widget/playbackcontrols/playbackcontrols.cpp

@@ -57,7 +57,7 @@ PlaybackControls::PlaybackControls(QWidget *parent)
 	lower_left_layout->setContentsMargins(0, 0, 0, 0);
 
 	cur_tc_lbl_ = new RationalSlider();
-	cur_tc_lbl_->set_display_type(RationalSlider::k_time);
+	cur_tc_lbl_->set_display_type(slider::k_time);
 	cur_tc_lbl_->set_minimum(0);
 	connect(cur_tc_lbl_, &RationalSlider::value_changed, this,
 			&PlaybackControls::time_changed);