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.cpp

@@ -28,14 +28,14 @@
 #include "common/qtutils.h"
 #include "common/xmlutils.h"
 #include "core.h"
-#include "dialog/progress/progress.h"
 #include "node/color/ociobase/ociobase.h"
 #include "node/factory.h"
+#include "node/group/group.h"
+#include "node/project/folder/folder.h"
 #include "node/serializeddata.h"
 #include "pluginSupport/olivehost.h"
 #include "ofxhPluginCache.h"
 #include "render/diskmanager.h"
-#include "window/mainwindow/mainwindow.h"
 
 namespace olive
 {