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/node/audio/volume/volume.cpp

@@ -21,7 +21,7 @@
 
 #include "volume.h"
 
-#include "widget/slider/floatslider.h"
+#include "node/sliderdisplaytype.h"
 
 namespace olive
 {
@@ -39,7 +39,7 @@ VolumeNode::VolumeNode()
 	add_input(k_volume_input, NodeValue::k_float, 1.0);
 	set_input_property(k_volume_input, QStringLiteral("min"), 0.0);
 	set_input_property(k_volume_input, QStringLiteral("view"),
-					 FloatSlider::k_decibel);
+					 slider::k_decibel);
 
 	set_flag(k_audio_effect);
 	set_effect_input(k_samples_input);