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/project.h

@@ -27,9 +27,10 @@
 #include <QUuid>
 
 #include "node/output/viewer/viewer.h"
+#include "node/project/folder/folder.h"
 #include "node/project/footage/footage.h"
+#include "node/project/sequence/sequence.h"
 #include "node/color/colormanager/colormanager.h"
-#include "window/mainwindow/mainwindowlayoutinfo.h"
 
 namespace olive
 {