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)

tests/gtest/node_generator_test.cpp

@@ -171,7 +171,7 @@ TEST(MatrixGenerator, InputDefaultsAndProperties)
 	EXPECT_EQ(node->get_input_property(olive::MatrixGenerator::k_scale_input,
 									 QStringLiteral("view"))
 				  .toInt(),
-			  int(olive::FloatSlider::k_percentage));
+			  int(olive::slider::k_percentage));
 	EXPECT_EQ(node->get_input_property(olive::MatrixGenerator::k_scale_input,
 									 QStringLiteral("min"))
 				  .value<QVector2D>(),
@@ -676,7 +676,7 @@ TEST(NoiseGenerator, InputDefaults)
 	EXPECT_EQ(node->get_input_property(olive::NoiseGeneratorNode::k_strength_input,
 									 QStringLiteral("view"))
 				  .toInt(),
-			  int(olive::FloatSlider::k_percentage));
+			  int(olive::slider::k_percentage));
 
 	EXPECT_EQ(int(node->get_input_data_type(olive::NoiseGeneratorNode::k_color_input)),
 			  int(olive::NodeValue::k_boolean));